Engine Controls - Self Diagnostics - 4.3L: Overview Chevrolet Silverado 1500 HD

MODEL IDENTIFICATION

Vehicle model is identified by the fifth character of Vehicle Identification Number (VIN). VIN is stamped on metal pad on top of left end of instrument panel, near windshield. See MODEL IDENTIFICATION table.

MODEL IDENTIFICATION

Description

The Diagnostic System Check is an organized approach to identifying a condition that is created by a malfunction in the powertrain control system. The Diagnostic System Check must be the starting point for any driveability concern. The Diagnostic System Check directs the service technician to the next logical step in order to diagnose the concern. Understanding and correctly using the diagnostic table reduces diagnostic time, and prevents the replacement of good parts.

Test Description

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 Lack of communication may be caused by a partial or a total malfunction of the Class 2 serial data circuit. The specified procedure determines the particular condition.
2. 5 This step stores the Powertrain Control Module (PCM) Diagnostic Trouble Code (DTC) information into the scan tool's memory. After you complete the diagnostic procedure, review the captured information in order to catch the next DTC if the control module stores multiple DTCs. Review the Freeze Frame data and the Failure Records data. Use this information in order to determine how frequently and how recently the DTC set. This information may help diagnose an intermittent condition. Information about the operating conditions at the time that the DTC set may also help diagnose an intermittent condition. Capturing the stored information saves the data that the PCM loses during the following conditions: When a diagnostic procedures instructs you to clear the DTCs. When a diagnostic procedure instructs you to disconnect the PCM connectors. When a diagnostic procedure instructs you to replace the PCM. See appropriate REMOVAL & INSTALLATION article.
3. 6 The presence of DTCs which begin with "U", indicate that some other module is not communicating. Following the specified procedure will gather all the available information before you perform the tests.
4. 8 If there are other modules with DTCs set, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-158208-S06912544392003091700000) . The DTC list directs you to the appropriate diagnostic procedure. If the control module stores multiple powertrain DTCs, diagnose the DTCs in the following order: Component level DTCs, such as sensor DTCs, solenoid DTCs, and relay DTCs. Diagnose the multiple DTCs within this category in numerical order. Begin with the lowest numbered DTC, unless the diagnostic table directs you otherwise. System level DTCs, for example, misfire DTCs, fuel trim DTCs, and catalyst DTCs.
5. 10 This step is for areas that have inspection and maintenance testing procedures for emissions testing. Use this step if the testing facility found one or more I/M system status that did not set.

The numbers below refer to the step numbers in the diagnostic procedure.

1. 3 Lack of communication may be due to a partial malfunction of the class 2 serial data circuit or due to a total malfunction of the class 2 serial data circuit. The specified procedure will determine the particular condition.
2. 4 Determine if the Powertrain Control Module (PCM) has DTCs set which may affect engine cooling operation.
3. 5 The presence of DTCs which begin with "U" indicate some other module is not communicating. The specified procedure will compile all the available information before tests are performed.

Several states require that a vehicle pass on-board diagnostic (OBD) system tests and the I/M emission inspection in order to renew license plates. This is accomplished by viewing the I/M System Status display on a scan tool. Using a scan tool, the technician can observe the I/M System Status in order to verify that the vehicle meets the criteria that complies with the local area requirements.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 1 Any DTCs set, even those that are not listed in the INSPECTION/MAINTENANCE SYSTEM DTCS table, may prevent the required DTCs from running. If there is any question as to whether a set DTC is disabling the required I/M diagnostic, review the Conditions for Running in the diagnostic procedures for the DTC required by the I/M diagnostic. A list of disabling DTCs, if applicable, is contained in the supporting text for that DTC.
2. 2 Anytime a control module is reprogrammed or the diagnostic trouble codes are cleared as part of a repair procedure, all the I/M System Status indicators will reset to NO.
3. 3 Use discretion when determining whether the entire system set procedure needs to be performed. For example, if the only tests that have not run are those that require the engine to be at operating temperature, then only those individual tests need to be run. There is no need to allow the engine to completely cool in order to run these tests.

The purpose of the I/M Complete System Set Procedure is to satisfy the enable criteria necessary to execute all of the I/M readiness diagnostics, and complete the trips for those particular diagnostics. When all diagnostic tests are complete, the I/M System Status indicators are set to YES. Perform this test when more than one or all of the I/M System Status indicators are set to NO.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 1 Make sure that you perform the I/M System Check before performing this test. Failure to do so may result in difficulty updating the status to YES.
2. 2 This step runs the HO2S Heater Tests and initiates the EVAP System Test. Preprogramming the scan tool will reduce the amount of time the oxygen sensor heaters operate while verifying the enable criteria. The engine control module considers the engine to be cold if the following conditions are met: Engine Coolant Temperature (ECT) less than 86°F (30°C). ECT and Intake Air Temperature (IAT) are within 14°F (8°C) of each other at start-up.
3. 3 This step runs the EVAP, AIR and the Oxygen Sensor Tests. The EVAP Test begins once the engine coolant reaches a calibrated temperature. The AIR Test, if equipped, begins shortly after Closed Loop and the indicated speed is achieved. The Oxygen Sensor Tests begin once the engine is at operating temperature, in Closed Loop fuel control, and a calibrated amount of time has elapsed.
4. 4 This step runs the Exhaust Gas Recirculation (EGR) Tests (if equipped). The EGR Tests are run during a gradual deceleration with a closed throttle. The vehicle speed is required in order to maintain a high, steady MAP signal.
5. 5 This step runs the Catalyst Tests. This test runs during the idle period immediately following a cruise period that meets a minimum calibrated RPM and time period.
6. 6 Perform the individual system test for any of the systems that do not update to YES.
7. 7 The I/M System Status only reports on whether or not a diagnostic has run, not the outcome of the test. If any emission related DTC sets after the tests are complete, the DTC will require diagnosis.

The purpose of this test is to satisfy the enable criteria necessary to execute I/M readiness diagnostics for the catalyst system. The test may be used to set the I/M System Status indicators to YES. Ensure that the vehicle meets the requirements listed in Conditions for Running before performing this test. Failure to meet the necessary requirements may produce inaccurate test results.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 1 Make sure that you perform the I/M System Check before performing this test. Failure to do so may result in difficulty updating the status to YES.
2. 2 The Catalyst Test during the idle period immediately following the cruise period.
3. 3 This step identifies a first failure of a type "B" DTC. A DTC only appears on the I/M System Status display when the DTC becomes a MIL illuminating DTC. This occurs on the second failure of a type "B" DTC. A first failure of a type "B" DTC will not allow the I/M System Status to update to YES. See «DIAGNOSTIC AIDS»(ref-158208-S31543744172003091700000) .
4. 4 This step helps identify any unique or unusual criteria required to run the diagnostic test if the universal set procedure does not. This information is located in Conditions for Running DTC.
5. 5 The I/M System Status only reports on whether or not a diagnostic has run, not the outcome of the test. If any Emission Related DTC sets after the tests are complete, the DTC will require diagnosis.

The purpose of this test is to satisfy the enable criteria necessary to execute I/M readiness diagnostics for the evaporative emission (EVAP) system. The test may be used to set the I/M System Status indicators to YES. Service Bay Tests are included on the scan tool for some systems depending upon vehicle make and model. The test is designed to allow the EVAP Diagnostic Tests to run in service bay conditions. Ensure that the vehicle meets the requirements listed in Conditions for Running before performing either EVAP System Test. Failure to meet the necessary requirements may produce inaccurate test results.

The purpose of this test is to satisfy the enable criteria necessary to execute I/M readiness diagnostics for the oxygen sensor (O2S, HO2S) system. The test may be used to set the I/M System Status to YES. Ensure the vehicle meets the requirements listed in Conditions for Running before performing this test. Failure to meet the necessary requirements may produce inaccurate test results.

The numbers below refer to the step numbers on the procedures.

1. 1 Make sure that you perform the I/M System Check before performing this test. Failure to do so may result in difficulty updating the status to YES.
2. 2 The oxygen sensor tests begin shortly after the indicated speed is achieved. The engine RPM may be too low in overdrive on manual transmission vehicles. If difficulty is encountered updating the status, operate the vehicle in the recommended gear during the test.
3. 3 This step identifies a first failure of a type "B" DTC. A DTC only appears on the I/M System Status display when the DTC becomes a MIL illuminating DTC. This occurs on the second failure of a type "B" DTC. A first failure of a type "B" DTC will not allow the I/M System Status to update to YES. See «DIAGNOSTIC AIDS»(ref-158208-S08329386222003091700000) .
4. 4 This step helps identify any unique or unusual criteria required to run the diagnostic test if the universal set procedure does not. This information is located in Conditions for Running DTC.
5. 5 The I/M System Status only reports on whether or not a diagnostic has run, not the outcome of the test. If any Emission Related DTC sets after the tests are complete, the DTC will require diagnosis.

The purpose of this test is to satisfy the enable criteria necessary to execute I/M readiness diagnostics for the Heated Oxygen Sensor (HO2S) system. The test may be used to set the I/M System Status to YES. Ensure that the vehicle meets the requirements listed in Conditions for Running before performing this test. Failure to meet the necessary requirements may produce inaccurate test results.

The numbers below refer to the step numbers on the procedures.

1. 1 Make sure that you perform the I/M System Check before performing this test. Failure to do so may result in difficulty updating the status to YES.
2. 2 Preprogramming the scan tool will reduce the amount of time the oxygen sensor heaters operate while verifying the enable criteria.
3. 3 This step identifies a first failure of a type "B" DTC. A DTC only appears on the I/M System Status display when the DTC becomes a MIL illuminating DTC. This occurs on the second failure of a type "B" DTC. A first failure of a type "B" DTC will not allow the I/M System Status to update to YES. See «DIAGNOSTIC AIDS»(ref-158208-S33150187082003091700000) .
4. 4 This step helps identify any unique or unusual criteria required to run the diagnostic test if the universal set procedure does not. This information is located in Conditions for Running DTC.
5. 5 The I/M System Status only reports on whether or not a diagnostic has run, not the outcome of the test. If any emission related DTC sets after the tests are complete, the DTC will require diagnosis.

Battery positive voltage is supplied directly to the Malfunction Indicator Lamp (MIL). The Powertrain Control Module (PCM) turns the MIL ON by grounding the MIL control circuit. There should be a steady Malfunction Indicator Lamp (MIL) with the ignition ON and the engine OFF.

MIL Operation

The Malfunction Indicator Lamp (MIL) is located on the instrument panel.

MIL Function

1. The MIL informs the driver that a malfunction has occurred and the vehicle should be taken in for service as soon as possible.
2. The MIL illuminates during a bulb test and a system test.
3. A Diagnostic Trouble Code (DTC) will be stored if a MIL is requested by the PCM.

MIL Illumination

1. The MIL will illuminate with the ignition ON and the engine not running.
2. The MIL will turn OFF when the engine is started.
3. The MIL will remain ON if the self-diagnostic system has detected a malfunction.
4. The MIL may turn OFF if the malfunction is not present.
5. If the MIL is illuminated and then the engine stalls, the MIL will remain illuminated so long as the ignition switch is ON.
6. If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition switch is cycled OFF, then ON.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 4 This step tests for a short to voltage on the MIL control circuit. With the fuse removed there should be no voltage on the MIL control circuit.

Battery positive voltage is supplied directly to the Malfunction Indicator Lamp (MIL). The Powertrain Control Module (PCM) turns the MIL ON by grounding the MIL control circuit.

The Malfunction Indicator Lamp (MIL) is located on the instrument panel.

MIL Function

1. The MIL informs the driver that a malfunction has occurred and the vehicle should be taken in for service as soon as possible.
2. The MIL illuminates during a bulb test and a system test.
3. A Diagnostic Trouble Code (DTC) will be stored if a MIL is requested by the diagnostic.

MIL Illumination

1. The MIL will illuminate with the ignition ON and the engine not running.
2. The MIL will turn OFF when the engine is started.
3. The MIL will remain ON if the self-diagnostic system has detected a malfunction.
4. The MIL may turn OFF if the malfunction is not present.
5. If the MIL is illuminated and then the engine stalls, the MIL will remain illuminated so long as the ignition switch is ON.
6. If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition switch is cycled OFF, then ON.

The number below refers to the step number in the diagnostic procedures.

1. 2 This step determines if the condition is with the MIL control circuit or the PCM.

The Powertrain Control Module (PCM) supplies a 12-volt reference circuit and a low reference circuit to both the Crankshaft Position (CKP) sensor and the Camshaft Position (CMP) sensor. The CKP sensor sends a signal to the PCM with each revolution of the crankshaft. The CMP sensor sends a signal to the PCM with each revolution of the camshaft. This Diagnostic Trouble Code (DTC) monitors the CKP signal and the CMP signal to determine if they are synchronized. If both signals are not observed by the PCM within a narrow period of time, the PCM will determine that an error has occurred and DTC P0016 will set.

The Mass Air Flow (MAF) sensor is an air flow meter that measures the amount of air entering the engine. The Powertrain Control Module (PCM) uses the MAF sensor signal in order to provide the correct fuel delivery for a wide range of engine speeds and loads. A small quantity of air entering the engine indicates a deceleration or idle. A large quantity of air entering the engine indicates an acceleration or high load condition. The MAF sensor has an ignition 1 voltage circuit, a ground circuit, and a signal circuit. The PCM applies a voltage to the sensor on the signal circuit. The sensor uses the voltage in order to produce a frequency based on inlet air flow through the sensor bore. The frequency varies within a range of around 2,000 Hertz at idle to about 10,000 Hertz at maximum engine load. The PCM uses the following sensor inputs in order to calculate a predicted MAF value

1. The Manifold Absolute Pressure (MAP).
2. The Intake Air Temperature (IAT).
3. The Engine Coolant Temperature (IAT).
4. The engine speed (RPM).

The PCM compares the actual MAF sensor frequency signal to the predicted MAF value. This comparison will determine if the signal is stuck based on a lack of variation, or is too low or too high for a given operating condition. DTC P0101 sets if the actual MAF sensor frequency signal is not within a predetermined range of the calculated MAF value.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 5 This step will determine if the MAP sensor pressure is within the proper range for a given altitude.
2. 6 This step will determine if the MAP sensor voltage is within the proper range at idle.
3. 7 This step will determine if the MAP sensor is operating properly to the change in manifold pressure.
4. 8 This step will determine if the TP sensor is operating properly.
5. 9 This step will determine if any mechanical faults have caused this DTC to set.
6. 10 This voltage drop test will determine if high resistance has caused this DTC to set.

Scheme 346

Scheme 347

ALTITUDE VS. BAROMETRIC PRESSURE

The Mass Air Flow (MAF) sensor is an air flow meter that measures the amount of air entering the engine. The Powertrain Control Module (PCM) uses the MAF sensor signal in order to provide the correct fuel delivery for a wide range of engine speeds and loads. A small quantity of air entering the engine indicates a deceleration or idle. A large quantity of air entering the engine indicates an acceleration or high load condition. The MAF sensor has an ignition 1 voltage circuit, a ground circuit, and a signal circuit. The PCM applies a voltage to the sensor on the signal circuit. The sensor uses the voltage in order to produce a frequency based on inlet air flow through the sensor bore. The frequency varies within a range of around 2,000 Hertz at idle to about 10,000 Hertz at maximum engine load. DTC P0102 sets if the PCM detects a frequency signal lower than the possible range of a properly operating MAF sensor.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 5 This step will determine if any mechanical faults have caused this DTC to set.
2. 7 This voltage drop test will determine if high resistance has caused this DTC to set.
3. 9 This step verifies the signal circuit from the MAF sensor electrical connector to the PCM.
4. 10 This step tests the signal circuit of the MAF sensor for a short to another 5 volt reference circuit.
5. 12 This step will determine which portion of the circuit or which component is shorted to ground.
6. 15 This step verifies that the signal circuit is not shorted to any other PCM circuit.

Scheme 348

Scheme 349

Scheme 350

The Mass Air Flow (MAF) sensor is an air flow meter that measures the amount of air entering the engine. The Powertrain Control Module (PCM) uses the MAF sensor frequency signal in order to provide the correct fuel delivery for a wide range of engine speeds and loads. A small quantity of air entering the engine indicates a deceleration or idle. A large quantity of air entering the engine indicates an acceleration or high load condition. The MAF sensor has an ignition 1 voltage circuit, a ground circuit, and a signal circuit. The PCM applies a voltage to the sensor on the signal circuit. The sensor uses the voltage in order to produce a frequency based on inlet air flow through the sensor bore. The frequency varies within a range of around 2,000 Hertz at idle to about 10,000 Hertz at maximum engine load. DTC P0103 sets if the PCM detects a frequency signal higher than the possible range of a properly operating MAF sensor.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 3 This step tests for Electromagnetic Interference (EMI) on the signal circuit of the MAF sensor. A frequency reading with the MAF sensor disconnected indicates an EMI related fault or a poor connection at the PCM. Disconnecting the MAF sensor may set additional related DTCs.
2. 4 This step will determine if incorrect harness routing has caused this DTC to set.
3. 5 This step will determine if water intrusion has caused this DTC to set.

Scheme 351

The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has the following circuits

1. 5-volt reference circuit.
2. Low reference circuit.
3. MAP sensor signal circuit.

The Powertrain Control Module (PCM) supplies 5 volts to the MAP sensor on the 5-volt reference circuit. The PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to the PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. The PCM should detect a high signal voltage at a high MAP, such as the ignition is ON, with the engine OFF, or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric (BARO) pressure. This occurs when the ignition switch is turned ON, with the engine OFF. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. The PCM calculates a predicted value for the MAP sensor based on throttle position and engine speed. The PCM then compares the predicted value to the actual MAP sensor signal. The DTC P0106 will set if the MAP sensor signal is not within the predicted range.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 5 This step tests the MAP sensors ability to correctly indicate barometric pressure.
2. 7 This step tests the MAP sensors ability to respond to an increase in engine vacuum.
3. 9 This step tests for a proper MAP sensor pressure with applied vacuum.

Scheme 352

Scheme 353

Scheme 354

The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has the following circuits

1. 5 volt reference circuit.
2. Low reference circuit.
3. MAP sensor signal circuit.

The Powertrain Control Module (PCM) supplies 5 volts to the MAP sensor on the 5 volt reference circuit. The PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to the PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. The PCM should detect a high signal voltage at a high MAP, such as the ignition is ON, with the engine OFF, or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric (BARO) pressure. This occurs when the ignition switch is turned ON, with the engine OFF. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. If the PCM detects a MAP sensor signal voltage that is excessively low, DTC P0107 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 4 Operate the vehicle within the same conditions as when the DTC failed. If you cannot duplicate the DTC, the information included in the Freeze Frame/Failure Records can aid in locating an intermittent condition.
2. 5 This step determines if voltage is available to the sensor. It also determines if there is sufficient current flow in the circuit.

Scheme 355

Scheme 356

The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has the following circuits

1. 5 volt reference circuit.
2. Low reference circuit.
3. MAP sensor signal circuit.

The Powertrain Control Module (PCM) supplies 5 volts to the MAP sensor on the 5 volt reference circuit. The PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to the PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. The PCM should detect a high signal voltage at a high MAP, such as the ignition is ON, with the engine OFF, or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric (BARO) pressure. This occurs when the ignition switch is turned ON, with the engine OFF. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. If the PCM detects a MAP sensor signal voltage that is excessively high, DTC P0108 will set.

The number below refers to the step number in the diagnostic procedures.

1. 4 Operate the vehicle within the same conditions as when the DTC failed. If you cannot duplicate the DTC, the information included in the Freeze Frame/Failure Records data can aid in locating an intermittent condition.

Scheme 357

Scheme 358

The Intake Air Temperature (IAT) sensor is a variable resistor, sometimes called a thermistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The Powertrain Control Module (PCM) supplies 5 volts to the IAT signal circuit. When the IAT sensor is cold, the sensor resistance is high. When the air temperature increases, the sensor resistance lowers. With high sensor resistance, the PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the IAT signal circuit. If the PCM detects an excessively low IAT signal voltage, indicating a high temperature, DTC P0112 sets.

The Intake Air Temperature (IAT) sensor is a variable resistor, sometimes called a thermistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The Powertrain Control Module (PCM) supplies 5 volts to the IAT signal circuit. When the IAT sensor is cold, the sensor resistance is high. When the air temperature increases, the sensor resistance lowers. With high sensor resistance, the PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the IAT signal circuit. If the PCM detects an excessively high IAT signal voltage, indicating a low temperature, DTC P0113 sets.

The Engine Coolant Temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The Powertrain Control Module (PCM) supplies 5 volts to the signal circuit and a ground for the ECT low reference circuit. When coolant temperatures are low, resistance is high. When coolant temperatures are high, the resistance is low. The PCM uses this input for engine controls and enabling criteria for diagnostics. The PCM will record the amount of time the engine is OFF. At restart, the PCM will compare the temperature difference between the ECT and Intake Air Temperature (IAT). Before failing this the test the PCM will perform a calculation to determine the presence of a block heater. If the temperature difference is not within the calculated amount, after the ignition OFF time, this DTC will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 3 This step tests for excessive resistance in the ECT sensor circuit.
2. 4 This step tests for excessive resistance in the IAT sensor circuit.
3. 5 This step tests for a skewed sensor through the range of temperatures affecting this DTC.

Scheme 359

Scheme 360

SENSOR TEMPERATURE VS. RESISTANCE

The Engine Coolant Temperature (ECT) sensor is a variable resistor, sometimes called a thermistor, that measures the temperature of the engine coolant. The Powertrain Control Module (PCM) supplies 5 volts to the ECT signal circuit and a ground for the ECT low reference circuit. When the ECT is cold, the sensor resistance is high. When the ECT increases, the sensor resistance lowers. With high sensor resistance, the PCM detects a high voltage on the ECT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the ECT signal circuit. If the PCM detects an excessively low ECT signal voltage, which is a high temperature indication, this diagnostic trouble code (DTC) will set.

The Engine Coolant Temperature (ECT) sensor is a variable resistor, sometimes called a thermistor, that measures the temperature of the engine coolant. The ECT sensor has a signal circuit and a low reference circuit. The Powertrain Control Module (PCM) supplies 5 volts to the ECT signal circuit and a ground for the ECT low reference circuit. When the ECT is cold, the sensor resistance is high. When the ECT increases, the sensor resistance lowers. With high sensor resistance, the PCM detects a high voltage on the ECT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the ECT signal circuit. If the PCM detects an excessively high ECT signal voltage, which is a low temperature indication, DTC P0118 sets.

The Throttle Position (TP) sensor is used by the Powertrain Control Module (PCM) to determine the throttle plate angle for various engine management systems. The TP sensor is a potentiometer type sensor with the following circuits

1. A 5-volt reference circuit.
2. A low reference circuit.
3. A signal circuit.

The PCM provides the TP sensor with a 5-volt reference circuit and a low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to the Wide Open Throttle (WOT) position provides the PCM with a signal voltage from less than one volt to more than 4 volts through the TO sensor signal circuit. When the conditions for running this Diagnostic Trouble Code (DTC) are met, the PCM will use the Manifold Absolute Pressure (MAP) sensor in order to determine if the predicted operating range of the TP sensor is correct. If the PCM detects the TP sensor voltage is out of the predicted range, DTC P0121 sets.

The Throttle Position (TP) sensor is used by the Powertrain Control Module (PCM) in order to determine the throttle plate angle for various engine management systems. The TP sensor is a potentiometer type sensor with 3 circuits

1. A 5-volt reference circuit.
2. A low reference circuit.
3. A signal circuit.

The PCM provides the TP sensor with a 5-volt reference circuit and a ground on the low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to the Wide Open Throttle (WOT) position provides the PCM with a signal voltage from less than 1 volt to more than 4 volts through the TP sensor signal circuit. If the PCM detects an excessively low signal voltage, this diagnostic trouble code (DTC) will set.

The number below refers to the step number in the diagnostic procedure.

1. 5 This step determines if voltage is available to the sensor. It also determines if there is sufficient current flow in the circuit.

Scheme 361

Scheme 362

The Throttle Position (TP) sensor is used by the Powertrain Control Module (PCM) to determine the throttle plate angle for various engine management systems. The TP sensor is a potentiometer type sensor with 3 circuits

1. A 5-volt reference circuit.
2. A low reference circuit.
3. A signal circuit.

The PCM provides the TP sensor with a 5-volts on the 5 volt reference circuit and a ground on the low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to the Wide Open Throttle (WOT) position provides the PCM with a signal voltage from less than 1 volt to more than 4 volts through the TP sensor signal circuit. If the PCM detects an excessively high signal voltage, this DTC will set.

The number below refers to the step number in the diagnostic procedure.

1. 6 Each terminal in the connector must be jumpered to its respective mate in the sensor. This allows the sensor to operate and permits access to the low reference circuit for the voltage drop measurement.

Scheme 363

Scheme 364

The Engine Coolant Temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the Powertrain Control Module (PCM) for engine control, and as an enabling criteria for some diagnostics.

Air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the Closed Loop temperature. If the coolant temperature does not increase normally or does not reach Closed Loop temperature, the diagnostics that use engine coolant temperature as enabling criteria may not run when expected.

This DTC will only run once per ignition cycle within the enabling conditions. If the PCM detects the calibrated amount of air flow and engine run time have been met and the ECT has not met the Closed Loop temperature, DTC P0125 sets.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 3 This step tests for excessive resistance in the ECT circuit.
2. 7 This step tests for a skewed sensor through the range of temperatures affecting this DTC.

Scheme 365

An Engine Coolant Temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the Powertrain Control Module (PCM) for engine control and as an enabling criteria for some diagnostics. The air flow coming into the engine is accumulated and used to determine if the engine has been driven within conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach regulating temperature of the thermostat, diagnostics that use engine coolant temperature as enabling criteria, may not run when expected. This diagnostic trouble code (DTC) will only run once per ignition cycle within the enabling conditions. If the engine coolant fails to reach a preset target temperature before a calculated air flow is accumulated, DTC P0128 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 3 This step tests for excessive resistance in the ECT circuit.
2. 7 This step tests for a skewed sensor through the range of temperatures affecting this DTC.

Scheme 366

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The powertrain control module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in the open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and closed loop is achieved the HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream; low HO2S voltage indicates a lean exhaust stream. If the PCM detects an HO2S voltage that stays below a specified value, DTCs P0131 or P0151 will set.

The number below refer to the step number in the diagnostic procedures.

1. 2 If the voltage is varying above and below the specified range, the condition is not present.

Scheme 367

Scheme 368

Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The Powertrain Control Module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and closed loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream; low HO2S voltage indicates a lean exhaust stream. If the PCM detects an HO2S voltage that stays above a specified value, DTCs P0132 or P0152 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 If the voltage is varying above and below the specified range, the condition is not present.

Scheme 369

Scheme 370

Scheme 371

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The powertrain control module (PCM) supplies the HO2S with a reference, or bias voltage of about 450 mV. When the engine is first started the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and closed loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream, low HO2S voltage indicates a lean exhaust stream. This diagnostic will only run once per ignition cycle. The PCM monitors the rich-to-lean and lean-to-rich transition time. A transition is defined as, the HO2S voltage changes from above 600 mV to below 200 mV or from below 200 mV to above 600 mV. If the PCM detects that the transition time is too long, DTCs P0133 & P0153 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 If the voltage is varying above and below the specified value, the condition is not present.

Scheme 372

Scheme 373

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The Powertrain Control Module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and closed loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream, low HO2S voltage indicates a lean exhaust stream. In the PCM detects that the HO2S voltage remains within the bias voltage range, DTCs P0134 & P0154 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 If the voltage is varying above and below the specified value, the condition is not present.

Scheme 374

Scheme 375

The Heated Oxygen Sensor (HO2S) must reach operating temperature to provide an accurate voltage signal. A heating element inside the HO2S minimizes the time required for the sensor to reach operating temperature. Voltage is provided to the heater by the Ignition 1 voltage circuit through a fuse. With the engine running, ground is provided to the heater by the HO2S heater low control circuit, through a low side driver within the Powertrain Control Module (PCM). The PCM commands the heater ON or OFF to maintain a specific HO2S operating temperature range. The PCM determines the temperature by measuring the current flow trough the heater. When the heater is in the ON state, the PCM will pulse the heater OFF for a duration of 50 ms, once per second. When the heater is in the OFF state, the PCM will pulse the heater ON for a duration of 50 ms, once per second. The PCM also monitors the current flow through the heater for diagnosis. This diagnostic will only run once per ignition cycle. If the PCM detects that the current is above or below a specified range, DTCs P0135, P0141, P0155 or P0161 sets.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 7 With no fault present, the test lamp will blink once per second.

Scheme 376

Scheme 377

Scheme 378

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The Powertrain Control Module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and closed loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream; low HO2S voltage indicates a lean exhaust stream. If the PCM detects an HO2S voltage that stays below a specified value, DTCs P0137 or P0157 will set.

The number below refers to the step number in the diagnostic procedure.

1. 2 If the voltage does not change more than the specified value, the condition is present.

Scheme 379

Scheme 380

Scheme 381

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The Powertrain Control Module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and closed loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream; low HO2S voltage indicates a lean exhaust stream. If the PCM detects an HO2S voltage that stays above a specified value, DTCs P0138 or P0158 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 If the voltage does not change more than the specified value, the condition is present.

Scheme 382

Scheme 383

Scheme 384

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The Powertrain Control Module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first starts the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and closed loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream; low HO2S voltage indicates a lean exhaust stream. This diagnostic will only run once per ignition cycle. If the PCM detects that the HO2S voltage remains within the bias voltage range, DTCs P0140 & P0160 will set.

The number below refers to the step number in the diagnostic procedures.

1. 2 If the voltage above and below the specified value, the condition is not present.

Scheme 385

Scheme 386

The Powertrain Control Module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled differently during Open and Closed Loop. During Open Loop the PCM determines fuel delivery based on sensor signals, without oxygen sensor input. During Closed Loop, the oxygen sensor inputs are added and used by the PCM to calculate short and long term fuel trim (fuel delivery adjustments). If the oxygen sensors indicate a lean condition, fuel trim values will be above 0 percent. If the oxygen sensors indicate a rich condition, fuel trim values will be below 0 percent. Short term fuel trim values change rapidly in response to the Heated Oxygen Sensor (HO2S) voltage signals. Long term fuel trim makes coarse adjustments in order to maintain air/fuel ratio of 14.7:1. If the PCM detects an excessively lean condition, this diagnostic trouble code (DTC) will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 5 If conditions were not corrected, see «BASIC DIAGNOSTIC PROCEDURES - 4.3L SIERRA & SILVERADO»(ref-152743) article.
2. 6 If conditions were not corrected, a worn cam, worn intake or exhaust valves, or other engine mechanical failures may be at fault.

Scheme 387

Scheme 388

The Powertrain Control Module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled differently during Open and Closed Loop. During Open Loop the PCM determines fuel delivery based on sensor signals, without oxygen sensor input. During Closed Loop, the oxygen sensor inputs are added and used by the PCM to calculate short and long term fuel trim, fuel delivery adjustments. If the oxygen sensors indicate a lean condition, fuel trim values will be above 0 percent. If the oxygen sensors indicate a rich condition, fuel trim values will be below 0 percent. Short term fuel trim values change rapidly in response to the Heated Oxygen Sensor (HO2S) voltage signals. Long term fuel trim makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. The fuel trim diagnostic will conduct a test to determine if a rich failure actually exists or if excessive vapor from the evaporative emission (EVAP) canister is causing a rich condition. If the PCM detects an excessively rich condition, this diagnostic trouble code (DTC) will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 5 If conditions were not corrected, see «BASIC DIAGNOSTIC PROCEDURES - 4.3L SIERRA & SILVERADO»(ref-152743) article.
2. 6 An EVAP canister that is saturated will cause a rich condition. If the conditions were not corrected, a worn cam, worn intake or exhaust valves, or other engine mechanical failure may be the problem.

Scheme 389

Scheme 390

The control module enables the appropriate fuel injector pulse for each cylinder. Ignition voltage is supplied to the fuel injectors. The control module controls each fuel injector by grounding the control circuit via a solid state device called a driver. The control module monitors the status of each driver. If the control module detects an incorrect voltage for the commanded state of the driver, a fuel injector control DTC sets.

The control module enables the fuel pump relay when the ignition switch is turned ON. The control module will disable the fuel pump relay within 2 seconds unless the control module detects ignition reference pulses. The control module continues to enable the furl pump relay as long as ignition reference pulses are detected. The control module disables the fuel pump relay within 2 seconds if ignition reference pulses cease to be detected and the ignition remains ON.

The Powertrain Control Module (PCM) uses information from the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring variations in the crankshaft rotation speed for each cylinder, the PCM is able to detect individual misfire events. A misfire rate that is high enough can cause 3-way catalytic converter damage. The Malfunction Indicator Lamp (MIL) will flash ON and OFF when the conditions for catalytic converter damage are present. If the PCM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 If the actual CKP variation values are not within the learned values, the misfire counters may increment.

Scheme 391

Scheme 392

Scheme 393

The Crankshaft Position (CKP) system variation learn feature is used to calculate reference period errors caused by slight tolerance variation in the crankshaft and the CKP sensors. The calculated error allows the Powertrain Control Module (PCM) to accurately compensate for reference period variations. This enhances the ability of the PCM to detect misfire events over a wider range of engine speed and load conditions. The PCM stores the CKP system variation values after a learn procedure has been performed. If the PCM detects the CKP system variation values are not stored in the PCM memory, DTC P0315 sets.

The knock sensor (KS) produces an AC voltage at all engine speeds and loads. The Powertrain Control Module (PCM) then adjusts the spark timing based on the amplitude and frequency of the KS signal. The PCM uses the KS signal to calculate the average voltage. Then the PCM assigns a voltage value. The PCM checks the KS and related wiring by comparing the actual knock signal to the assigned voltage range. A normal KS signal should stay within the assigned voltage range. This diagnostic trouble code (DTC) will set if the PCM malfunctions in a manner that will not allow proper diagnosis of the KS system.

The Powertrain Control Module (PCM) monitors a Knock Sensor (KS) in order to determine if detonation is present. The KS produces an AC voltage at all engine speeds and loads. The Powertrain Control Module (PCM) then adjusts the spark timing based on the amplitude and frequency of the KS signal. The PCM uses the KS signal to calculate the average voltage and than assigns a voltage range value. The PCM should monitor the KS signal within the assigned voltage range. This Diagnostic Trouble Code (DTC) will set if the KS signal is outside the assigned voltage range or the KS signal is not present.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 This step ensures the malfunction is present
2. 3 This step tests the KS for proper operation.
3. 6 If the KS signal wire is shorted to ground or shorted to voltage the KS may still produce a signal.
4. 7 If the KS low reference is shorted to ground or shorted to voltage the KS may still produce a signal.

Scheme 394

Scheme 395

The crankshaft position (CKP) sensor signal indicates the crankshaft speed and position. The CKP sensor is connected directly to the Powertrain Control Module (PCM), and consists of the following circuits

1. The 12-volt reference circuit.
2. The low reference circuit.
3. The CKP sensor signal circuit.

The crankshaft position (CKP) sensor signal indicates the crankshaft speed and position. The CKP sensor is connected directly to the Powertrain Control Module (PCM), and consists of the following circuits

1. The 12-volt reference circuit.
2. The low reference circuit.
3. The CKP sensor signal circuit.

If the PCM detects that the CKP sensor signal is incorrect for 3 seconds, DTC P0336 sets.

The Camshaft Position (CMP) Sensor is a hall-effect type sensor. The sensor produces one signal for each revolution of the camshaft in order to control the sequential fuel injection. The CMP sensor is designed to detect changes in a magnetic field. The Powertrain Control Module (PCM) supplies the CMP sensor with the following circuits

1. A 12 volt reference circuit.
2. A low reference circuit.
3. A signal circuit.

The CMP sensor produces a magnetic field whenever the ignition is ON. The CMP sensor is mounted near a reluctor wheel that is attached to the distributor shaft. When the distributor shaft rotates, and the reluctor wheel tooth passes by the CMP sensor, there is a change in the magnetic field. The CMP sensor converts each change in the magnetic field into a PULSE. If the PCM does not detect the CMP signal while the engine is running, Diagnostic Trouble Code (DTC) P0341 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 6 This step tests the CMP sensor signal circuit. Applying a voltage causes the CMP sensor high to low and low to high parameter top increase if the circuit and the PCM are operating properly.

Scheme 396

Scheme 397

The enhanced ignition system uses the crankshaft position (CKP) sensor in order to provide a timing input to the control module. Ignition control (IC) spark timing for each cylinder is based on this input. The control module provides the ignition timing signal to the ignition control module (ICM) to control the ignition coil. Each timing pulse detected by the ICM allows the ICM to energize the ignition coil. A large secondary ignition voltage is induced in the secondary coil by the primary coil. This high voltage is switched to the correct spark plug by the distributor. This diagnostic trouble code (DTC) will set if the Powertrain Control Module (PCM) detects an unusually high or low voltage on the ignition timing signal circuit.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 This step determines if the DTC is an intermittent.
2. 3 This step checks if the IC timing signal from the PCM is available at the ICM.

Scheme 398

Scheme 399

In order to maintain low emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx), the engine controls system uses a 3-Way Catalytic Converter (TWC). The catalyst within the TWC promotes a chemical reaction, which oxidizes the HC and CO that are present in the exhaust gas. This reaction converts these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx which converts into nitrogen. The Powertrain Control Module (PCM) monitors this process using Heated Oxygen Sensor (HO2S) bank 1, sensor 2, which is located in the exhaust stream past the TWC. The HO2S bank 1 sensor 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to effectively convert the exhaust emissions. If the catalyst is functioning correctly, the bank 1 of the HO2S 2 signal will be far less active than the signal that is produced by bank 1 of the HO2S. This indicates that the TWC oxygen storage capacity is below an acceptable threshold.

The PCM performs this diagnostic test at idle. When the conditions for running this DTC are met, the following occurs

1. The PCM captures the current rear HO2S rich-to-lean status.
2. The air-to-fuel ratio transitions from rich to lean or from lean to rich, depending on the captured rear HO2S rich-to-lean status.
3. The air-to-fuel ratio transitions a second time opposite the first air-to-fuel ratio transition.
4. The PCM captures the response time of the front and the rear HO2S when the air-to-fuel ratio transitions occur. The HO2S response time goes from less than 300 mV to more than 600 mV, and from more than 600 mV to less than 300 mV.
5. The PCM measures the time necessary for the rear HO2S voltage to cross a reference rich-to-lean threshold, minus the time necessary for the front HO2S voltage to cross the same rich-to-lean threshold. The difference between the oxygen storage capacity of the catalyst. This DTC sets if the time exceeds a predetermined threshold.

The number below refers to the step number in the diagnostic procedure.

1. 3 This step inspects for conditions that can cause the 3-way catalytic converter efficiency to appear degraded.
2. A catalytic converter which has been discolored may be due to an engine running rich, lean or had a previous sever misfire. Verifying the fuel trim percentages may be of assistance in determining if such a condition exists.

Scheme 400

This diagnostic tests the Evaporative Emission (EVAP) system for a small leak when the key is turned OFF and the correct conditions are met. Heat is transferred into a vehicle fuel tank while the vehicle is operating. When the vehicle is turned OFF, a change in the fuel tank vapor temperature occurs, which results in corresponding pressure changes in the fuel tank vapor space. This change is monitored by the control module using the fuel tank pressure sensor input. The control module then makes a judgement on the integrity of the system. With a 0.020 inch (0.51 mm) leak in the system, the amount of pressure change observed is significantly less than that of the sealed system. If the control module detects a pressure change less than a calibrated amount, DTC P0442 sets.

The numbers below refer to the step number in the diagnostic procedures.

1. 3 Introducing smoke in 15 second intervals may allow smaller leak areas to be more noticeable. When the system is less pressurized, the smoke will sometimes escape in a more condensed manner.
2. 5 This step verifies that repairs are complete and that no other condition is present.

Scheme 401

Scheme 402

An ignition voltage is supplied directly to the evaporative emission (EVAP) canister purge valve. The EVAP canister purge valve is pulse width modulated (PWM). The scan tool displays the amount of ON time as a percentage. The control module monitors the status of the driver. The control module controls the EVAP canister purge valve ON time by grounding the control circuit via an internal switch called a driver. If the control module detects an incorrect voltage for the commanded state of the driver, this DTC sets.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 This step tests if the concern is active. The EVAP solenoid is Pulse Width Module (PWM). You should hear a clicking sound when the purge solenoid is commanded to 50 percent. The clicking sound should stop when the EVAP purge solenoid is commanded to 0 percent. The rate at which the valve cycles should increase when the commanded state is increased, and decreased when the command state is decreased. Repeat the commands as necessary.
2. 5 This step verifies that the control module is providing ground to the EVAP purge solenoid.
3. 6 This step tests if a ground is constantly being applied to the EVAP purge solenoid.

Scheme 403

Scheme 404

This DTC tests the Evaporative Emission (EVAP) system for a restricted or blocked EVAP vent path. The control module commands the EVAP canister purge solenoid ON and the EVAP canister vent solenoid ON. This allows vacuum to be applied to the EVAP system. Once a calibrated vacuum level has been reached, the control module commands the EVAP canister purge solenoid OFF and the EVAP canister purge solenoid OFF. The control module monitors the Fuel Tank Pressure (FTP) sensor for a decrease in vacuum. If the vacuum does not decrease to near 0 inches H2O in a calibrated time, this DTC sets.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 3 This test determines if the failure is present or intermittent.

Scheme 405

Scheme 406

An ignition voltage is supplied directly to the Evaporative (EVAP) emissions vent valve. The control module controls the EVAP vent valve by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the controlled component. The control module monitors the status of the driver. If the control module detects an incorrect voltage for the commanded state of the driver, DTC P0449 sets.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 A click should be heard or felt when the valve operates. Ensure that both the ON and the OFF states are commanded. Repeat the commands as necessary.
2. 5 This step verifies that the control module is providing ground to the EVAP vent solenoid.
3. 6 This step tests if the EVAP vent solenoid control circuit is grounded.

Scheme 407

Scheme 408

The Fuel Tank Pressure (FTP) sensor measures the difference between the air pressure or vacuum in the Evaporative Emission (EVAP) system, and the outside air pressure. The control module supplies a 5-volt reference and a low reference circuit to the FTP sensor. The FTP sensor signal circuit voltage caries depending on EVAP system pressure or vacuum. If the FTP sensor signal voltage goes below a calibrated value, this DTC sets.

The number below refers to the step number in the diagnostic procedures.

1. 5 This step tests for the proper operation of the circuit in the high voltage range.
2. 6 The 5-volt reference circuit for the fuel tank pressure (FTP) is routed through connector C152. This area may provide a good test point for diagnosing concerns with this circuit.
3. 7 The FTP sensor signal circuit is routed through connector C152. This area may provide a good test point for diagnosing concerns with this circuit.

Scheme 409

Scheme 410

The Fuel Tank Pressure (FTP) sensor measures the difference between the air pressure or vacuum in the evaporative emission (EVAP) system, and the outside air pressure. The control module supplies a 5-volt reference and a low reference circuit to the FTP sensor. The FTP sensor signal circuit voltage caries depending on EVAP system pressure or vacuum. If the FTP sensor signal voltage increases above a calibrated value, this DTC sets.

The number below refers to the step number in the diagnostic procedures.

1. 2 If DTC P0641 or P0651 set, the 5-volt reference circuit maybe shorted to a voltage.

Scheme 411

Scheme 412

The control module tests the Evaporative Emission (EVAP) system for a large leak. The control module monitors the Fuel Tank Pressure (FTP) sensor signal to determine the EVAP system vacuum level. When the conditions are met, the control module commands the EVAP canister purge valve OPEN and the EVAP vent valve CLOSED. This allows engine vacuum to enter the EVAP system. At a calibrated time, or vacuum level, the control module commands the EVAP canister purge valve closed, sealing the system, and monitors the FTP sensor input in order to determine the EVAP system vacuum level. If the system is unable to achieve the calibrated vacuum level, or the vacuum level decreases too rapidly, this DTC sets.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 3 Introducing smoke in 15 second intervals may allow smaller leak areas to be more noticeable. When the system is less pressurized, the smoke will sometimes escape in a more condensed manner.
2. 5 This step verifies proper operation of the Fuel Tank Pressure (FTP) sensor.
3. 6 A normal operating FTP sensor should increase above 5 inches of H2O and stop between 6 and 7 inches of H2O.

Scheme 413

Scheme 414

Scheme 415

This DTC tests for undesired intake manifold vacuum flow to the Evaporative Emission (EVAP) system. The control module seals the EVAP system by commanding the EVAP canister purge valve OFF and the EVAP canister vent valve ON. The control module monitors the Fuel Tank Pressure (FTP) sensor to determine if a vacuum is being drawn on the EVAP system. If vacuum in the EVAP system is more than a predetermined value within a predetermined time, this DTC sets.

The engine idle speed is controlled by the Idle Air Control (IAC) valve. The IAC valve is on the throttle body. The IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The valve consists of a movable pintle, driven by a gear attached to a two phase bi-polar permanent magnet electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation or movement, called steps. The stepper motor has two separate windings that are called coils. Each coil is fed by two circuits from the Powertrain Control Module (PCM). When the PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition key is turned OFF. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If the engine idle speed is out of range for a calibrated period of time, an idle speed diagnostic trouble code (DTC) may set.

The number below refers to the step number in the diagnostic procedures.

1. 5 This test will determine the ability of the PCM and IAC valve control circuits to control the IAC valve.
2. 7 This test will determine the ability of the PCM to provide the IAC circuits with a ground. On a normal operating system, the test lamp should not flash while the IAC counts are incrementing.

Scheme 416

Scheme 417

Scheme 418

The engine idle speed is controlled by the Idle Air Control (IAC) valve. The IAC valve is on the throttle body. The IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The valve consists of a movable pintle, driven by a gear attached to a two phase bi-polar permanent magnet electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation or movement, called steps. The stepper motor has two separate windings that are called coils. Each coil is fed by two circuits from the Powertrain Control Module (PCM). When the PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition key is turned OFF. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If the engine idle speed is out of range for a calibrated period of time, an idle speed diagnostic trouble code (DTC) may set.

The number below refers to the step number in the diagnostic procedures.

1. 5 This test will determine the ability of the engine controller and IAC valve circuits to control the IAC valve.
2. 7 This test will determine the ability of the PCM to provide the IAC valve circuits with a ground. On a normally operating system, the test lamp should not flash while the IAC counts are incrementing.

Scheme 419

Scheme 420

Scheme 421

This diagnostic applies to internal microprocessor integrity conditions within the Powertrain Control Module (PCM). This diagnostic also addresses if the PCM is not programmed.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 These DTCs indicate that the PCM is not programmed.

Scheme 422

The Powertrain Control Module (PCM) provides 5 volts to the following sensors

1. The Manifold Absolute Pressure (MAP) sensor.
2. The Engine Oil Pressure (EOP) sensor.
3. The Throttle Position (TP) sensor.

These 5-volt reference circuits are independent of each other outside the PCM, but are bussed together inside the PCM. Therefore a circuit condition on one sensor 5-volt reference circuit may affect the other sensor 5-volt reference circuits. The PCM monitors the voltage on the 5-volt reference circuit. If the PCM detects the voltage is out of tolerance DTC P0641 sets.

The Malfunction Indicator Lamp (MIL) is located on the instrument panel cluster (IPC). The MIL informs the driver that an emission system fault has occurred and that the engine control system requires service. The control module monitors the MIL control circuit for conditions that are incorrect for the commanded state of the MIL. For example, a failure condition exists if the control module detects low voltage when the MIL is commanded OFF, or high voltage when the MIL is commanded ON. If the control module detects an improper voltage on the MIL control circuit, DTC P0650 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 4 This step tests for a short to ground in the MIL control circuit. With the PCM disconnected and the ignition ON the MIL should be OFF.
2. 5 This step tests for a short to voltage on the MIL control circuit. With the fuse removed there should be no voltage on the MIL control circuit.

Scheme 423

Scheme 424

The Powertrain Control Module (PCM) provides 5 volts to the following sensors

1. The Air Conditioning (A/C) pressure sensor.
2. The Fuel Tank Pressure (FTP) sensor, if equipped.

These 5-volt reference circuits are independent of each other outside the PCM, but are bussed together inside the PCM. Therefore a circuit condition on one sensor 5-volt reference circuit may affect the other sensor 5-volt reference circuits. The PCM monitors the voltage on the 5-volt reference circuit. If the PCM detects that the voltage is out of tolerance, DTC P0651 sets.

The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has the following circuits

1. 5-volt reference circuit.
2. Low reference circuit.
3. MAP sensor signal circuit.

The Powertrain Control Module (PCM) supplies 5 volts to the MAP sensor on the 5-volt reference circuit. The PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to the PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during idle or a deceleration. The PCM should detect a high signal voltage at a high MAP, such as the ignition is ON, with the engine OFF, or at Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when the ignition switch is turned ON, with the engine OFF. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. If the PCM detects a MAP sensor signal voltage that is intermittently high, DTC P1106 sets.

The number below refers to the step number in the diagnostic procedure.

1. 3 This step attempts to pinpoint the location of the intermittent fault.

Scheme 425

The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has the following circuits

1. 5-volt reference circuit.
2. Low reference circuit.
3. MAP sensor signal circuit.

The Powertrain Control Module (PCM) supplied 5 volts to the MAP sensor on the 5-volt reference circuit. The PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to the PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a high signal voltage at a low MAP, such as during an idle or a deceleration. The PCM should detect a high signal voltage at a high MAP, such as the ignition is ON, with the engine OFF, or at Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the barometric pressure (BARO). This occurs when the ignition switch is turned ON, with the engine OFF. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. If the PCM detects a MAP sensor signal voltage that is intermittently low, DTC P1107 sets.

The number below refers to the step number in the diagnostic procedure.

1. 3 This step attempts to pinpoint the location of the intermittent fault.

Scheme 426

The Intake Air Temperature (IAT) sensor is a variable resistor, sometimes called a thermistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The Powertrain Control Module (PCM) supplies 5 volts to the IAT signal circuit. When the IAT sensor is cold, the sensor resistance is high. When the air temperature increases, the sensor resistance lowers. With high sensor resistance, the PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the IAT signal circuit. If the PCM detects an intermittent high IAT signal voltage, indicating a low temperature, DTC P1111 sets.

The Intake Air Temperature (IAT) sensor is a variable resistor, sometimes called a thermistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The Powertrain Control Module (PCM) supplies 5 volts to the IAT signal circuit. When the IAT sensor is cold, the sensor resistance is high. When the air temperature increases, the sensor resistance lowers. With high sensor resistance, the PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the IAT signal circuit. If the PCM detects an intermittent low IAT signal voltage, indicating a high temperature, DTC P1112 sets.

The Engine Coolant Temperature (ECT) sensor is a variable resistor, sometimes called a thermistor, that measures the temperature of the engine coolant. The Powertrain Control Module (PCM) supplies 5 volts to the ECT signal circuit. When the ECT is cold, the sensor resistance is high. When the ECT increases, the sensor resistance lowers. With high sensor resistance, the PCM detects a high voltage on the ECT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the ECT signal circuit. If the PCM detects an excessively low ECT signal voltage, which is a high temperature indication, this diagnostic trouble code (DTC) will set.

The Engine Coolant Temperature (ECT) sensor is a variable resistor, sometimes called a thermistor, that measures the temperature of the engine coolant. The Powertrain Control Module (PCM) supplies 5 volts to the ECT signal circuit. When the ECT is cold, the sensor resistance is high. When the ECT increases, the sensor resistance lowers. With high sensor resistance, the PCM detects a high voltage on the ECT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the ECT signal circuit, if the PCM detects an excessively high signal voltage, which is a low temperature indication, this diagnostic trouble code (DTC) will set.

The Throttle Position (TP) sensor is used by the Powertrain Control Module (PCM) in order to determine the throttle plate angle for various engine management systems. The TP sensor is a potentiometer type sensor with 3 circuits

1. A 5-volt reference circuit.
2. A low reference circuit.
3. A signal circuit.

The PCM provides the TP sensor with 5 volts on the 5-volt reference circuit and a ground on the low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to the Wide Open Throttle (WOT) position provides the PCM with a signal through the TP sensor signal circuit. If the PCM detects an intermittent excessively high signal voltage, DTC P1121 sets.

The number below refers to the step number in the diagnostic procedure.

1. 7 This test will determine an intermittent faulty TP sensor utilizing the DMMs MIN MAX 100 millisecond capture mode.

Scheme 427

Scheme 428

The Throttle Position (TP) sensor is used by the PCM in order to determine the throttle plate angle for various engine management systems. The TP sensor is a potentiometer type sensor with 3 circuits

1. A 5-volt reference circuit.
2. A low reference circuit.
3. A signal circuit.

The PCM provides the TP sensor with 5 volts on the 5-volt reference circuit. Rotation of the TP sensor rotor from the closed throttle position to the Wide Open Throttle (WOT) position provides the PCM with a signal voltage from below 1.0 volt to greater than 4.0 volts through the TP sensor signal circuit. If the PCM detects an intermittent excessively low signal voltage, DTC P1122 sets.

The number below refers to the step number in the diagnostic procedure

1. 6 This test will determine an intermittent faulty TP sensor utilizing the DMMs MIN MAX, 100 millisecond capture mode.

Scheme 429

Scheme 430

Heated Oxygen Sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The Powertrain Control Module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperature and closed loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream; low HO2S voltage indicates a lean exhaust stream. This diagnostic will only run once per ignition cycle. The PCM monitors the number of rich-to-lean and lean-to-rich transitions. A transition is defined as, the HO2S voltage changes from above 600 mV to below 200 mV or from below 200 mV to above 600 mV. If the PCM detects that the number of transitions were less than a specified value, DTCs P1133 or P1153 will set.

The numbers below refer to the step numbers in the diagnostic procedures.

1. 2 If the voltage is varying above and below the specified value, the condition is not preset.

Scheme 431

Scheme 432

The PCM uses the ECT sensor to monitor the engine for an over temperature condition. This condition occurs when the coolant temperature is above 270°F (132°C). When an over temperature condition is present, DTC P1258 will set. The PCM will disable two groups of four cylinders by turning OFF the fuel injectors. By switching between the two groups of cylinders, the PCM is able to reduce the temperature of the coolant.

The Powertrain Control Module (PCM) detects engine misfire events by monitoring variations in the crankshaft rotation speed. Wheel speed changes caused by rough road conditions can cause changes in crankshaft rotation speed. By monitoring the wheel speed sensors, the Anti-lock Brake System (ABS) can determine if the vehicle is operating on a rough road condition severe enough to effect misfire detection, a rough road signal is sent to the PCM on the serial data circuit. If DTC P0300 is set and the rough road information is not available due to an ABS malfunction, DTC P1380 sill set.

The Powertrain Control Module (PCM) detects engine misfire events by monitoring variations in the crankshaft rotation speed. Wheel speed changes caused by rough road conditions can cause changes in crankshaft rotation speed. By monitoring the wheel speed sensors, the Anti-Lock Brake System (ABS) can determine if the vehicle is operating on a rough road. If the ABS is detecting a rough road condition severe enough to effect misfire detection, a rough road signal is sent to the PCM on the serial data circuit. If DTC P0300 is set and the rough road information is not available due to the ABS malfunction, DTC P1381 will set.

The number below refers to the step number in the diagnostic procedures.

1. 1 This step will diagnose a malfunction in the serial data circuits.

See also:
• SENSOR OPERATING RANGE CHARTS - TRUCKS