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Wiring diagrams Section: Testing & Diagnostics

Engine Controls Self-Diagnostics - 4.3L

Engine Control System Self-Diagnostics - 4.8L, 5.3L & 6.0L

Engine Control System Self-Diagnostics - 8.1L "C", "G" & "K" Series Trucks

Engine Controls - Strategy Based Engine Diagnosis

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Engine Controls Self-Diagnostics - 6.0L - Bi-Fuel & Cng: Overview GMC Sierra 2500

Testing & Diagnostics ~11032 words

Description

The Diagnostic System Check - Engine Controls is an organized approach to identifying a condition that is created by a malfunction in the engine 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 test procedure reduces diagnostic time and prevents the replacement of good parts.

Test Description

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

2 Lack of communication may be because of a partial or a total malfunction of the class 2 serial data circuit. The specified procedure determines the particular condition.
3 Lack of communication may be because of a partial or a total malfunction of the class 2 serial data circuit. The specified procedure determines the particular condition.
4 This step determines if the vehicle will start on the gasoline fuel system.
6 This step stores the Powertrain Control Module (PCM) Diagnostic Trouble Code (DTC) information into the scan tools 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/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.
7 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.
9 If there are other modules with DTCs set, see «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-151060-S20665527472002120600000) . 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 test directs you otherwise. System level DTCs, for example, misfire DTCs, fuel trim DTCs, and catalyst DTCs.
14 This step determines if the vehicle is operating on CNG. The Alternative Fuels Operation (AFO) signal command and AFO signal parameters will display AFECM if the vehicle is operating on CNG.
18 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 statuses that did not set.

The Malfunction Indicator Lamp (MIL) informs the driver that a malfunction has occurred and the vehicle should be taken in for service as soon as possible.

This vehicle utilizes a Powertrain Control Module (PCM) and an Alternative Fuels Engine Control Module (AF ECM) in order to control engine operation. Both modules need the ability to illuminate the MIL. In order to allow both modules to illuminate the MIL the AF ECM contains circuitry that will operate the MIL for both modules. The AF ECM will command the MIL ON whenever the PCM provides a ground path on the AF ECM MIL Input circuit.

When the ignition is first turned ON, the PCM will continuously illuminate the MIL for a bulb and system check. When the engine is started and operating the PCM will turn the MIL OFF if no MIL Request PCM DTCs are set.

The AF ECM connects into the base vehicle MIL Control circuit path and will command the MIL ON whenever a PCM MIL request or an alternative fuels DTC is set. Both the AF ECM and the PCM simultaneously test certain DTCs and may illuminate the MIL.

The instrument panel gauge fuse supplies the positive voltage source for the MIL. The PCM and/or the AF ECM can supply the ground path.

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

1 The System Check must be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in mis-diagnosis.
2 With the ignition ON and the engine OFF, the PCM should continuously illuminate the MIL. A 2 second bulb check is not considered continuously illuminated.
3 The scan tool will command BOTH the PCM and AF ECM to turn the MIL OFF. The scan tool will command both the PCM and the AF ECM to turn the MIL OFF.
4 This step checks if a PCM system failure is the cause of the inoperative MIL. The most likely cause of no MIL and a no-start is a loss of voltage to the control modules.
6 This step tests for an open MIL input circuit. The PCM should always command the MIL ON when the ignition is ON and the engine is OFF.
7 This step tests for an open MIL Control circuit or IP.

The Malfunction Indicator Lamp (MIL) informs the driver that a malfunction has occurred and the vehicle should be taken in for service as soon as possible.

The AF ECM is wired into the base vehicle MIL Control circuit path and will command the MIL ON whenever a PCM MIL request or an alternative fuels DTC is set. Both the AF ECM and the PCM simultaneously test certain DTCs and may illuminate the MIL.

The instrument panel gauge fuse supplies the positive voltage source for the MIL. The PCM and/or the AF ECM can supply the ground path.

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

1 The System Check MUST be performed before proceeding with this diagnostic test. Failure to perform the System Check will result in misdiagnosis.
2 With ignition on, with engine off, the Powertrain Control Module (PCM) should continuously illuminate the MIL.
3 The scan tool will command BOTH the PCM and the AF ECM to turn the MIL OFF.
5 This step tests if the MIL input circuit is requesting MIL illumination. There should not be a MIL input while the MIL is commanded OFF.

The Mass Air Flow (MAF) sensor is an air flow meter. The MAF sensor is supplied ignition voltage and a ground. The MAF sensor heats a wire grid or a small wire within the sensor to a predetermined temperature. As inlet air flows across and cools the wire grid, the MAF sensor must increase current flow to the wire grid in order to maintain a constant wire grid temperature. The MAF sensor converts the required wire grid current flow into a frequency signal. The Powertrain Control Module (PCM) and the AF ECM monitor the MAF signal circuit frequency and determine the flow and mass of the air entering the engine.

The PCM and the AF ECM convert the MAF signal circuit frequency into a grams per second value. During low air flow rates, such as at engine idle, the MAF sensor will produce a low frequency signal, around 2000 Hz or 6 grams per second. During high air flow rates, such as at wide open throttle-road load, the MAF sensor will produce a high frequency signal, around 9000 Hz or 125 grams per second.

The PCM and the AF ECM monitor the actual MAF sensor air flow rate and compare the actual air flow rate to a calculated air flow rate. The calculated air flow rate is based upon engine speed and Barometric (BARO) pressure. The PCM and the AF ECM use the MAP sensor in order to determine the BARO when the ignition switch is turned ON and the engine is NOT cranked. This BARO reading may also be updated whenever the engine is operated at wide open throttle.

This DTC is used to indicate that the actual MAF sensor air flow rate does not match a calculated air flow rate.

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

4 This step verifies that the MAP sensor voltage is not too low. The AF ECM uses the MAP sensor value in order to calculate the predicted MAF sensor flow.
5 This step verifies that the MAP sensor voltage is not too high. The AF ECM uses the MAP sensor value in order to calculate the predicted MAF sensor flow.
6 This step compares the MAP sensor kPa value to a known-good vehicle.
7 The step tests the MAP sensors ability to respond to a change in the engine vacuum when the engine is started.
8 This step verifies that the TP sensor is not the cause of this DTC. The AF ECM uses the TP sensor value in order to calculate the predicted MAF sensor flow.
9 Any un-metered air entering the engine may cause this DTC to set. This step eliminates any conditions which can cause a DTC to set with a normally operating MAF sensor.
10 A high resistance in the ignition voltage circuit of the MAF sensor may cause this DTC to set.

This DTC is used to indicate a MAF sensor frequency that is below the normal operating range of the sensor.

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

2 This step determines if the condition is present.
3 This step determines if the condition affects the Powertrain Control Module (PCM).
4 This step determines if the proper MAF Signal circuit voltage is available from the PCM.
8 This step tests for proper MAF sensor ignition voltage and ground circuits.
9 This step determines if the MAF sensor ignition voltage circuit or the MAF sensor ground circuit is the cause of the condition.

This DTC is used to indicate a MAF sensor frequency that is above the normal operating range of the sensor.

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

2 This step determines if the condition is present.
3 This step determines if the condition affects the PCM.
4 This step determines if the MAF sensor is generating the high frequency signal.
5 This step determines if the AF ECM is generating the high frequency signal.
6 This step determines if an external source is inducing a high frequency to the MAF Signal circuit.

The Manifold Absolute Pressure (MAP) sensor is a pressure sensor, located on top of intake manifold. The PCM supplies about 5 volts on the MAP sensor reference voltage circuit. The PCM also supplies a ground circuit to the MAP sensor. When manifold vacuum is low the MAP signal voltage rises to near 4.5 volts. As manifold vacuum increases the MAP signal voltage drops to around 1 volt. The PCM and the AF ECM monitor the MAP signal circuit voltage and determine the vacuum within the intake manifold.

Certain vehicle models will also use the MAP sensor in order to determine the barometric pressure when the ignition switch is turned ON and the engine is NOT cranked. This BARO reading may also be updated whenever the engine is operated at wide open throttle.

The Throttle Position (TP) sensor is a potentiometer. The Powertrain Control Module (PCM) supplies about 5 volts on the TP sensor reference voltage circuit. The PCM also supplies a ground circuit to the TP sensor. When the throttle is depressed the TP signal voltage rises to near the supplied 5 volts. As the throttle is released, the TP signal voltage drops to around 0.6 volt. The PCM and AF ECM monitor the TP signal circuit voltage and determine the angle (or opening) of the throttle blade.

This DTC is used to indicate a MAP sensor signal voltage that does not change when TP sensor voltage changes.

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

2 This step tests the MAP sensors ability to correctly indicate barometric pressure. In order to determine the correct MAP sensor display of barometric pressure for any elevation perform either of the following: Compare the recorded MAP sensor display value to the BARO display on a known-good vehicle. Obtain the local weather service barometer reading (in. Hg). Multiply the barometer reading by 3.4. The result indicates the correct BARO value for your area.
3 This step tests the MAP sensors ability to correctly indicate engine vacuum.
7 This step indicates this sensor is currently functioning correctly. If you were sent to this diagnostic procedure from other service information return to that procedure now.

The Manifold Absolute Pressure (MAP) sensor is a pressure sensor, located on top of intake manifold. The Powertrain Control Module (PCM) supplies about 5 volts on the MAP sensor reference voltage circuit. The PCM also supplies a ground circuit to the MAP sensor. When manifold vacuum is low the MAP signal voltage rises to near 4.5 volts. As manifold vacuum increases the MAP signal voltage drops to around 1 volt. The PCM and Alternative Fuels Engine Control Module (AF ECM) monitor the MAP signal circuit voltage and determine the vacuum within the intake manifold.

This DTC is used to indicate a MAP sensor signal voltage that is less than the normal operating range of the sensor.

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

2 This step determines if the condition is present.
3 This step determines if the condition affects the PCM.
4 This step determines if the MAP sensor is the cause of the condition.
5 This step determines if the 5-volt reference circuit is the cause of the condition. 4.8 volts is the minimum output that the PCM should supply on the 5-volt reference circuit.

The Manifold Absolute Pressure (MAP) sensor is a pressure sensor, located on top of intake manifold. The Powertrain Control Module (PCM) supplies about 5 volts on the MAP sensor reference voltage circuit. The PCM also supplies a ground circuit to the MAP sensor. When manifold vacuum is low the MAP signal voltage rises to near 4.5 volts. As manifold vacuum increases the MAP signal voltage drops to around 1 volt. The PCM and the Alternative Fuels Engine Control Module (AF ECM) monitor the MAP signal circuit voltage and determine the vacuum within the intake manifold.

This DTC is used to indicate a MAP sensor signal voltage that is greater than the normal operating range of the sensor.

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

2 This step determines if the condition is present.
3 This step determines if the condition affects the PCM.
4 This step determines if the MAP sensor is the shorted condition.
5 This step determines if an open sensor ground condition is present.
6 This step determines if the 5-volt reference circuit is shorted to a voltage. 5.2 volts is the maximum output that the PCM should supply on the 5-volt reference circuit.

The Intake Air Temperature (IAT) sensor is a thermistor, located in air cleaner resonator box. The IAT thermistor has high resistance when cold and low resistance when hot. The Powertrain Control Module (PCM) supplies about 5 volts on the IAT signal circuit. The PCM also supplies a ground circuit to the IAT sensor. When IAT resistance is high (cold sensor), the IAT signal voltage remains near the supplied 5 volts. As the IAT sensor warms and resistance drops, more signal circuit voltage is pulled lower to the sensor ground. The PCM and the AF ECM monitor the IAT signal circuit voltage in order to determine the temperature of the air entering the engine.

This DTC is used to indicate an IAT signal circuit voltage that is less than the normal operating range of the sensor.

IAT sensor is integral to MAF sensor.

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

2 This step determines if the condition is present.
4 This step determines if the IAT sensor is shorted.
5 This step determines if a short is present in the wiring harness.
6 This step determines if the PCM or the AF ECM is the cause of the shorted condition.
7 This step determines if an open circuit condition is present between the PCM and the signal circuit splice. If an open condition exists in this circuit the PCM will indicate a high signal voltage, while the AF ECM will indicate low signal voltage.

The Intake Air Temperature (IAT) sensor is a thermistor. The IAT thermistor has high resistance when cold and low resistance when hot. The Powertrain Control Module (PCM) supplies about 5 volts on the IAT signal circuit. The PCM also supplies a ground circuit to the IAT sensor. When IAT resistance is high the IAT signal voltage remains near the supplied 5 volts. As the IAT sensor warms and resistance drops, more signal circuit voltage is pulled lower to the sensor ground. The PCM and the AF ECM monitor the IAT signal circuit voltage in order to determine the temperature of the air entering the engine.

This DTC is used to indicate an IAT signal circuit voltage that is greater than the normal operating range of the sensor.

IAT sensor is integral to MAF sensor.

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

2 This step determines if the condition is present.
3 This step determines if the condition affects the PCM.
4 This step determines if the IAT sensor is the open condition.
5 This step determines if the sensor ground circuit is the open condition.
6 This step determines if the IAT signal circuit wiring is the open condition.
11 If the connector terminals pass inspection test the sensor signal circuit for a short to a 5-volt reference circuit. Although an unlikely failure, this condition should be diagnosed before replacing the PCM.

The Engine Coolant Temperature (ECT) sensor is a thermistor. The ECT thermistor has high resistance when cold and low resistance when hot. The Powertrain Control Module (PCM) supplies about 5 volts on the ECT signal circuit. The PCM also supplies a ground circuit to the ECT sensor. When ECT resistance is high the ECT signal voltage remains near the supplied 5 volts. As the ECT sensor warms and resistance drops, more signal circuit voltage is pulled lower to the sensor ground. The PCM and the AF ECM monitor the ECT signal circuit voltage in order to determine the temperature of the engines.

This DTC sets when the ECT signal circuit voltage is below the normal operating range of the sensor.

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

2 This step determines if the failure condition is present.
5 This step determines if the ECT sensor is shorted.
6 This step determines if a short is present in the wiring harness.
7 This step determines if the PCM or the AF ECM is the cause of the shorted condition.
8 This step determines if an open circuit condition is present between the PCM and the signal circuit splice. If an open condition exists in this circuit the PCM should indicate high signal voltage (a high signal DTC should be set) while the AF ECM would indicate low signal voltage (this DTC).

The Engine Coolant Temperature (ECT) sensor is a thermistor. The ECT thermistor has high resistance when cold and low resistance when hot. The Powertrain Control Module (PCM) supplies about 5 volts on the ECT signal circuit. The PCM also supplies a ground circuit to the ECT sensor. When ECT resistance is high (cold sensor), the ECT signal voltage remains near the supplied 5 volts. As the ECT sensor warms and resistance drops, more signal circuit voltage is pulled lower to the sensor ground. The PCM and the Alternative Fuels Engine Control Module (AF ECM) monitor the ECT signal circuit voltage in order to determine the engines temperature.

This DTC is used to indicate an ECT signal circuit voltage that is greater than the normal operating range of the sensor.

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

2 This step determines if the condition is present.
3 This step determines if the condition affects the PCM.
4 This step determines if the ECT sensor is the open condition.
5 This step determines if the sensor ground circuit is the open condition.
6 This step determines if the ECT signal circuit wiring is the open condition.
9 If the connector terminals pass inspection test the sensor signal circuit for a short to a 5-volt reference circuit. Although an unlikely failure, this condition should be diagnosed before replacing the PCM.

The Throttle Position (TP) sensor is a potentiometer. The Powertrain Control Module (PCM) supplies about 5 volts on the TP sensor reference voltage circuit. The PCM also supplies a ground circuit to the TP sensor. When the throttle is depressed the TP signal voltage rises to near the supplied 5 volts. As the throttle is released the TP signal voltage drops to around 0.6 volt. The PCM and the Alternative Fuels Engine Control Module (AF ECM) monitor the TP signal circuit voltage and determine the angle or opening of the throttle blade.

This DTC is used to indicate a throttle angle that is greater than calculated, based upon engine speed and engine MAP.

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

2 The step verifies that a PCM specific MAP sensor condition is not the cause of the DTC. The MAP sensor display should change when the engine is started. The MAP value should change with the engine speed. If a MAP sensor voltage changes, there is no condition with the MAP sensor, or PCM wiring.
3 The step verifies that a PCM specific TP sensor condition is not the cause of the DTC. The TP sensor value should change with the engine speed. If the TP sensor voltage changes, there is no condition with the TP sensor.
4 The step verifies that a AF ECM specific MAP sensor condition is not the cause of the DTC. The MAP sensor display should change when the engine is started. The MAP value should change with the engine speed. If a MAP sensor voltage changes, there is no condition with the MAP sensor, or AF ECM wiring.
5 The step verifies that a specific TP sensor condition is not the cause of the DTC. The TP sensor value should change with the engine speed. If the TP sensor voltage changes, there is no condition with the TP sensor.
8 This step determines if the AF ECM or wiring is the short to voltage.
9 This step tests the 5-volt reference circuit, signal circuit, and the PCM. If the scan tool displays 5 volts, the TP sensor 5-volt reference and the TP sensor signal circuits are okay.
10 This step tests the TP sensor ground circuit. If the test lamp does not illuminate, test the ground circuit for an open or high resistance.

The Throttle Position (TP) sensor is a potentiometer. The Powertrain Control Module (PCM) supplies about 5 volts on the TP sensor reference voltage circuit. The PCM also supplies a ground circuit to the TP sensor. When the throttle is depressed the TP signal voltage rises to near the supplied 5 volts. As the throttle is released the TP signal voltage drops to around 0.6 volt. The PCM and the Alternative Fuels Engine Control Module (AF ECM) monitor the TP signal circuit voltage and determine the angle (or opening) of the throttle blade.

This DTC is used to indicate a TP sensor signal voltage that is less than the normal operating range of the sensor.

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

2 This step determines if the failure condition is present.
3 This step determines if the TP signal is going low only when the throttle is opened.
4 This step determines if the failure condition affects the PCM.
5 This step determines if the TP sensor is the cause of the condition.
6 This step determines if the 5-volt reference circuit is the cause of the condition. 4.8 volts is the minimum output that the PCM should supply on the 5-volt reference circuit.

The Throttle Position (TP) sensor is a potentiometer. The Powertrain Control Module (PCM) supplies about 5 volts on the TP sensor reference voltage circuit. The PCM also supplies a ground circuit to the TP sensor. When the throttle is depressed the TP signal voltage rises to near the supplied 5 volts. As the throttle is released the TP signal voltage drops to around 0.6 volt. The PCM and the Alternative Fuels Engine Control Module (AF ECM) monitor the TP signal circuit voltage and determine the angle or opening of the throttle blade.

This DTC is used to indicate a TP sensor signal voltage that is greater than the normal operating range of the sensor.

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

2 This step determines if the condition is present.
3 This step determines if the TP signal is going high only when the throttle is opened.
4 This step determines if the condition affects the PCM.
5 This step determines if the TP signal circuit is the shorted condition.
6 This step determines if an open sensor ground circuit is the cause of the condition.
7 This step determines if the 5-volt reference circuit is shorted to a voltage. 5.2 volts is the maximum voltage that the PCM should supply on the 5-volt reference circuit.

The Engine Coolant Temperature (ECT) sensor is a thermistor. The ECT thermistor has high resistance when cold and low resistance when hot. The Powertrain Control Module (PCM) supplies about 5 volts on the ECT signal circuit. The PCM also supplies a ground circuit to the ECT sensor. When ECT resistance is high (cold sensor), the ECT signal voltage remains near the supplied 5 volts. As the ECT sensor warms and resistance drops, more signal circuit voltage is pulled lower to the sensor ground. The PCM and the Alternative Fuels Engine Control Module (AF ECM) monitor the ECT signal circuit voltage in order to determine the engines temperature.

This DTC is used to indicate an engine coolant temperature that is less than the temperature necessary to enable AF ECM Closed Loop fuel control.

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

2 Ensure that the engine can reach proper operating temperature in a timely manner before continuing with ECT sensor diagnosis.
3 This step determines if the AF ECM can monitor an open ECT signal circuit (high voltage).
4 This step determines if the AF ECM can monitor a shorted ECT signal circuit (low voltage).
5 This step determines if the ECT sensor is accurately monitoring engine coolant temperature.

The Heated Oxygen Sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 600°F (360°C) to produce voltage. The HO2S will produce more voltage when the exhaust is rich and less voltage when the exhaust is lean. The HO2S operating voltage range is around 0.10 volt (100 mV) to 1 volt (1000 mV).

The Powertrain Control Module (PCM) supplies a bias, or reference, voltage to the HO2S. This bias voltage is around 450 millivolts.

This DTC is used to indicate an HO2S signal voltage that is less than normal operating range of the sensor.

The Powertrain Control Module (PCM) supplies a bias (or reference) voltage to the HO2S. This bias voltage is around 450 millivolts.

This DTC is used to indicate an HO2S signal voltage that is greater than the normal operating range of the sensor.

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

3 An HO2S contaminated by silicone will have a White, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual cause of silica contamination is the use of un-approved silicone RTV engine gasket material or the use of silicone based sprays or fluids within the engine. If the cause of this contamination is not corrected, the replacement HO2S will also get contaminated.
8 An HO2S contaminated by silicone will have a White, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual cause of silica contamination is the use of un-approved silicone RTV engine gasket material or the use of silicone based sprays or fluids within the engine. If the cause of this contamination is not corrected, the replacement HO2S will also get contaminated.

The Heated Oxygen Sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean, the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 600°F (360°C) to produce voltage. The HO2S will produce more voltage when the exhaust is rich and less voltage when the exhaust is lean. The HO2S operating voltage range is between 0.10 volt (100 mV) to 1 volt (1000 mV).

The PCM supplies a bias (or reference) voltage to the HO2S. This bias voltage is around 450 mV. The AF ECM monitors the bias voltage as well as the voltage produced by the HO2S.

During normal Closed Loop fuel control operation, the AF ECM will add fuel when the HO2S indicates a lean exhaust content. When the HO2S indicates a rich exhaust content, the AF ECM will subtract fuel. This oscillation above and below the bias voltage, sometimes referred to as activity or switching can be monitored with the HO2S signal voltage.

This DTC is used to indicate an HO2S with inactive signal voltage.

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

3 This step determines if proper sensor activity is present. When in Closed Loop fuel control, the HO2S voltage should rapidly vary between more than and less than the bias voltage.
6 This step determines if the PCM and the high and low circuits between the control module and the HO2S connector are operating properly.
9 This step determines if the HO2S heater circuit operates correctly up to the HO2S connector.
10 This step determines if the proper circuit resistance exists between the HO2S Low circuit and control module ground.
15 This step determines if the proper circuit resistance exists between the HO2S low circuit and control module ground with a cold sensor. A loose HO2S or faulty thread to exhaust electrical contact causes higher resistance when the HO2S is cold. Although 500 ohms is allowed, the typical resistance should be less than 50 ohms.

The Heated Oxygen Sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean, the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 600°F (360°C) to produce voltage. The HO2S will produce more voltage when the exhaust is rich and less voltage when the exhaust is lean. The HO2S operating voltage range is between 0.10 volt (100 mV) to 1 volt (1000 mV).

The PCM supplies a bias or reference voltage to the HO2S. This bias voltage is around 450 mV. The AF ECM monitors the bias voltage as well as the voltage produced by the HO2S.

During normal closed loop fuel control operation, the AF ECM will add fuel (enriches the mixture) when the HO2S indicates a lean exhaust content. When the HO2S indicates a rich exhaust content, the AF ECM will subtract fuel. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with a scan tool.

The oxygen sensor contains a heater. The heater is necessary in order to quickly warm the sensor to operating temperature. The heater will also maintain operating sensor temperature during extended idle conditions. A functioning oxygen sensor heater will allow the sensor to produce voltage in a short amount of time. A sensor at operating temperature will cause the HO2S signal voltage to go above or below, depending upon exhaust oxygen content, the bias voltage.

This DTC is used to indicate an HO2S signal voltage that did not leave the cold sensor bias voltage range in an acceptable amount of time.

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

2 As the heater warms the oxygen sensor to operating temperature, the HO2S will output a voltage in relation to the amount of oxygen in the exhaust. With the ignition ON and the engine OFF, the exhaust usually contains a high oxygen content. With the engine OFF, the HO2S voltage will usually decrease from the initial bias voltage due to the high oxygen content.
5 This step tests for a proper HO2S heater ignition feed and ground supply.
6 This step tests for proper control module and signal circuit operation.

The Heated Exhaust Oxygen Sensor (HO2S) is used to determine oxygen content of exhaust. Oxygen content indicates when engine is operating lean or rich. When engine is operating lean, exhaust will have more oxygen content. Once at operating temperature, HO2S will produce a voltage in relation to oxygen content of exhaust. HO2S requires an operating temperature of 600°F (360°C) to produce voltage. HO2S will produce more voltage when exhaust is rich and less voltage when exhaust is lean. HO2S operating voltage range is 0.10-1 volt (100-1000 mV).

Alternate Fuel Engine Control Module (AF ECM) uses closed loop air/fuel metering system when HO2S is at operating temperature. During normal closed loop fuel control operation, AF ECM will add fuel (enriches mixture) when HO2S indicates lean exhaust content. When HO2S indicates rich exhaust content, AF ECM will subtract fuel (lean out mixture). This addition and subtraction to air/fuel mixture may be indicated on scan tool as long and short term fuel trim values. Short term values are adjustments to fuel mixture that occur quickly. Long term values are adjustments to fuel mixture that accumulate over longer periods of time. Long and short term fuel trim values are displayed on scan tool as percent. Positive percentage values indicate fuel is being added and negative percentage values indicate fuel is being subtracted from the mixture.

This DTC is used to indicate a fully rich fuel trim system that cannot compensate for a lean engine exhaust content.

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

2 This step determines whether the fault is present.
4 If DTC P0171 and DTC P0174 is set at the same time, this indicates both banks of the engine are operating lean. Inspect the items that would cause both banks to operate lean.
5 A vacuum leak causes DTC P0171 and DTC P0174 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the PCV valve for being the correct one for this application. Make sure that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.

This DTC is used to indicate a fully lean fuel trim system that cannot compensate for a rich engine exhaust content.

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

2 This step determines whether the fault is present.
4 If DTC P0172 and DTC P0175 set at the same time, then both banks of the engine are operating rich. Inspect items that would cause both banks to operate rich.

The Heated Oxygen Sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean, the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 600°F (360°C) to produce voltage. The HO2S will produce more voltage when the exhaust is rich, and less voltage when the exhaust is lean. The HO2S operating voltage range is between 100 mV to 1000 mV.

The AF ECM utilizes a Closed Loop air/fuel metering system when the HO2S is at operating temperature. During normal Closed Loop fuel control operation, the AF ECM will add fuel when the HO2S indicates a lean exhaust content. When the HO2S indicates a rich exhaust content, the AF ECM will subtract fuel. This addition and subtraction to the air/fuel mixture may be indicated on a scan tool as the Long and Short Term Fuel Trim values. Short Term Fuel Trim values are adjustments to the fuel mixture that occur quickly. Long Term Fuel Trim values are adjustments to the fuel mixture that accumulate over longer periods of time. The Long and Short Term Fuel Trim values are displayed on a scan tool as both percent and counts. Positive percentage values indicate that fuel is being added to the mixture. Negative percentage values indicate that fuel is being subtracted from the mixture.

This DTC indicates a fully rich fuel trim system that cannot compensate for a lean engine exhaust content.

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

2 This step determines whether the fault is present.
4 If DTC P0171 and DTC P0174 is set at the same time, this indicates both banks of the engine are operating lean. Inspect the items that would cause both banks to operate lean.
5 A vacuum leak causes DTC P0171 and DTC P0174 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the PCV valve for being the correct one for this application. Make sure that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.

The Heated Oxygen Sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean, the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 600°F (360°C) to produce voltage. The HO2S will produce more voltage when the exhaust is rich, and less voltage when the exhaust is lean. The HO2S operating voltage range is between 100 mV to 1000 mV.

This DTC is used to indicate a fully lean fuel trim system that cannot compensate for a rich engine exhaust content.

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

2 This step determines whether the fault is present.
4 If DTC P0172 and DTC P0175 set at the same time, then both banks of the engine are operating rich. Inspect items that would cause both banks to operate rich.

The Fuel Tank Temperature (FTT) sensor is a variable resistor that measures the temperature of the fuel in the CNG fuel tank. The AF ECM supplies 5 volts to the FTT signal circuit and supplies a ground to the low reference circuit. This DTC sets when the FTT signal voltage is below the normal operating range of the sensor.

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

4 Tests for the proper operation of the circuit in the low voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to voltage.
8 This step tests the signal circuit of the FTT sensor for a short to another 5.0 volt reference circuit.

The Fuel Rail Temperature (FRT) sensor is a variable resistor that measures the temperature of the fuel in the CNG fuel rail. The AF ECM supplies 5 volts to the FRT signal circuit and supplies a ground to the low reference circuit. This DTC sets when the FRT signal voltage is below the normal operating range of the sensor.

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

4 Tests for the proper operation of the circuit in the low voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to voltage.
8 This step tests the signal circuit of the FRT sensor for a short to another 5.0 volt reference circuit.

The Fuel Rail Pressure (FRP) sensor is a pressure sensor. The Alternative Fuels Engine Control Module (AF ECM) supplies about 5 volts on the FRP sensor reference voltage circuit. The AF ECM also supplies a ground circuit to the FRP sensor. When the fuel rail pressure is normal, the FRP signal voltage rises to near 2.5 volts. If the fuel rail pressure increases, the MAP signal voltage increases. The AF ECM monitors the FRP signal circuit voltage in order to determine the correct fuel injector pulse width modulation.

This DTC sets when the FRP sensor signal voltage is below the normal operating range of the sensor.

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

2 This step determines if the condition is present.
4 This step determines if the signal circuit of the FRP sensor is the cause of the condition.
5 This step determines if the 5-volt reference circuit is the cause of the condition.

This DTC sets when the FRP sensor signal voltage is more than the normal operating range of the sensor.

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

2 This step determines if the condition is present.
4 This step determines if an open sensor ground circuit condition is present. The FRP, FRT, FTP and FTT share a sensor ground. An open circuit between the splice and the AF ECM will cause all signal circuits to indicate high voltage.
5 This step determines if an open or shorted signal circuit is the cause of the condition.
6 This step determines if a shorted 5-volt reference circuit is the cause of the condition.
7 This step determines if an open sensor ground circuit between the harness connector of the FRP sensor and the splice is the cause of the condition.

The AF ECM enables the appropriate fuel injector on the intake stroke for each cylinder. An ignition voltage is supplied to the fuel injectors through a fuel injector relay. The AF ECM controls each fuel injector by grounding the control circuit via a solid state device called a driver. The AF ECM monitors the status of each driver. If the AF ECM detects an incorrect voltage for the commanded state of the driver, a fuel injector control DTC sets.

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

2 This step verifies that the condition is present
4 This step determines if the condition affects an entire bank of the engine. The cylinder pairings are 1, 3, 5, 7, and 2, 4, 6, 8.
5 This step tests for voltage at the fuel injector harness connector. The fuel injector relay supplies power to the coil side of the fuel injector harness connector.
6 This step verifies that the AF ECM is able to control the fuel injector. If the test lamp blinks, then the AF ECM and wiring are okay.
7 This step tests if a ground is constantly being applied to the fuel injector.
10 This step tests for voltage at the fuel injector relay coil. The IGN E fuse supplies power to the coil side of the fuel injector relay. If the fuse is open, a short to ground on the ignition supply circuit is indicated.
11 This step tests for voltage at the fuel injector relay switch. If the fusible link is open, a short to ground on the battery voltage supply circuit is indicated.
12 This step tests the ground circuit of the fuel injector relay. If the test lamp does not illuminate, repair the open circuit.

The Powertrain Control Module (PCM) monitors the Crankshaft Position (CKP) sensor signal in order to determine engine speed. The PCM signals engine speed to the Alternative Fuels Engine Control Module (AF ECM) by pulse-width-modulating the engine speed signal circuit.

This DTC is used to indicate no engine speed signal is being monitored by the AF ECM while the MAF sensor signal indicates the crankshaft is rotating.

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

2 This step determines if the condition is causing a no-start.
3 This step determines if the condition affects the PCM.
4 This step determines if the fault is present.

The Camshaft Position (CMP) sensor is a sensor designed to detect changes in a magnetic field. The Powertrain Control Module (PCM) supplies the CMP sensor with a 12-volt reference, a low reference, and 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 part of the camshaft. When the camshaft rotates the reluctor wheel interrupts the magnetic field. The CMP sensor converts each change in the magnetic field into a pulse. The PCM and the AF ECM use the CMP sensor signal in order to calculate the correct timing for sequential fuel injection. This DTC sets when the AF ECM does not detect a CMP signal while the engine is running.

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

2 This step determines if the fault is present.
4 This step determines if the condition affects the PCM. If the CMP high to low and low to high transition parameters increment, then the condition only affects the AF ECM.
5 This step tests the ignition feed circuit of the CMP sensor.
6 This step tests the low reference circuit of the CMP sensor.
7 This step tests the signal circuit of the CMP sensor. If the scan tool parameters increment when the test lamp contacts the signal circuit, then the wiring and modules are okay.
8 This step tests if the AF ECM is the cause of the condition. If the scan tool parameters increment when the AF ECM is disconnected, the signal circuit, and the PCM are okay.

The Powertrain Control Module (PCM) and the AF ECM monitor the Exhaust Gas Recirculation (EGR) valve pintle position. If the AF ECM detects an excessively low EGR valve position signal voltage, DTC P0405 will set.

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

2 This step verifies that a condition does not exist with the EGR valve 5-volt reference circuit. If DTC P1635 is set, refer to that diagnostic procedure first. See «DTC P1635: 5-VOLT REFERENCE 1 CIRCUIT»(ref-151060-S11736825112002121200000) .
3 This step verifies that the condition is present.
5 This step tests the 5-volt reference circuit of the EGR valve. If the DMM does not display 5 volts, test the circuit for an open.
6 This step tests the signal circuit of the EGR valve. The scan tool should display 5 volts with the jumper wire installed.
7 This step determines if the EGR valve is the cause of the concern. If the scan tool displays 5 volts in both data lists, inspect for poor connections at the EGR valve.
9 This step verifies that the AF ECM is not the cause of the condition.

The Powertrain Control Module (PCM) and the AF ECM monitor the Exhaust Gas Recirculation (EGR) valve pintle position. If the AF ECM detects an excessively high EGR valve position signal voltage, DTC P0406 will set.

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

2 This step verifies that the condition is present.
4 This step determines if the signal circuit of the EGR valve is shorted to a voltage.
5 This step tests the 5-volt reference circuit of the EGR valve. If the DMM displays above 5 volts, test the circuit for a short to voltage.
7 This step tests the low reference circuit of the EGR valve position sensor. If the test lamp does not illuminate, test the circuit for an open.
8 This step verifies that the PCM is not the cause of the condition.

The vehicle speed output signal is generated by the Powertrain Control Module (PCM). The signal is a duplicate of the Vehicle Speed Sensor (VSS) signal that the PCM receives from the VSS. The PCM contains an integrated circuit called a driver that switches the signal circuit to ground at a rate of 4000 pulses for every mph the vehicle is driven. Various modules (IP, Cruise, AF ECM, etc.) that are connected to the vehicle speed output circuit convert the 4K PPM into a vehicle speed value.

This DTC is used to indicate that the AF ECM cannot monitor an accurate vehicle speed.

The AF ECM communicates via the Class 2 serial data circuit. The AF ECM monitors this circuit for activity. If the AF ECM does not receive or send a message on the Class 2 circuit for 10 seconds, this DTC sets.

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

2 This step determines if the condition is present.
4 This step determines if the condition affects other control modules.
5 This step determines if the condition is causing a no-start.
6 This step determines if the AF ECM is the cause of the condition. If the scan tool displays other module information with the AF ECM disconnected, replace the AF ECM.
8 This step determines if the class 2 serial data circuit between the AF ECM and the splice pack is grounded. If the circuit is not grounded, see appropriate BODY CONTROL MODULES article in ACCESSORIES & EQUIPMENT in order to diagnose the rest of the class 2 circuits and modules.
9 This step determines if a fuse that supplies voltage to the AF ECM is open. If a fuse is open, test the circuit and all related components for a short to ground.

This diagnostic applies to internal microprocessor integrity conditions within the control module. This diagnostic also addresses if the control module is not programmed. The following DTCs are diagnosed in this DTC test

DTC P0601
DTC P0602
DTC P0603
DTC P0605
DTC P0606

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

2 A DTC P0602 indicates the AF ECM is not programmed.

The transmission on this vehicle has a Park Neutral Position (PNP) and back-up switch. This switch incorporates a transmission range selector switch. There are 4 inputs, "PRND A", "B", "C", "P", which are switches to ground. The 4 switches have an unique switch pattern. The Powertrain Control Module (PCM) and the AF ECM use the combination of the switch patterns in order to determine the gear selected by the driver. See TRANSMISSION RANGE SWITCH VALID INPUT COMBINATIONS table.

This diagnostic monitors the 4 inputs in order to determine what gear position the transmission is in. When the AF ECM detects that an illegal range has occurred, this DTC sets.

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

2 This step determines if the condition is present.
4 This step determines if the condition affects the PCM. If the Transmission Data display matches the selected transmission range, the condition only affects the AF ECM.
5 This step determines if the ground circuit of the TR switch is the cause of the condition. If the data displays HI for all of the circuits, test the ground circuit for an open.
6 When the TR switch is disconnected, all of the data parameters should display HI.
7 This step tests TR switch wiring for an open or the lack of the signal voltage from the PCM.
8 This step tests TR switch wiring and the PCM by providing a ground path through a fused jumper wire. When grounded, the scan tool range signal should change to LOW.
9 This step tests for an open circuit between the AF ECM and the splice.
10 This step determines if the AF ECM is the cause of the condition. If the data display switches from LOW to HI when the AF ECM is disconnected, the module is causing the short to ground.

The Alternative Fuels Engine Control Module (AF ECM) contains electronic devices called output driver modules. The output driver modules provide switchable outputs for operating solenoids, relays, telltales, and other devices. Each of the outputs has a status line that the AF ECM monitors.

In order for the status line to detect a failure, the state of the control circuit must be opposite of the commanded state. If the AF ECM is commanding an output ON, then 0 volts should be monitored by the status line. If the AF ECM is commanding an output OFF then the voltage supplied to the controlled device should be monitored by the status line.

This DTC sets when the AF ECM has detected a control circuit fault.

This DTC monitors the following low side driver circuits

The Alternative Fuels Operation (AFO) Signal control circuit.
The Low Pressure Lock-Off (LPL) solenoid control circuit.
The High Pressure Lock-Off (HPL) solenoid control circuit.

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

2 The AFO Signal data parameter is from the PCM and displays the observed state of the alternative fuels operation command circuit. The AFO Signal Command data parameter is from the AF ECM and displays the commanded state of the alternative fuels operation command circuit.
11 The AF ECM should only command the HPL solenoid ON for about 2 seconds. If the test lamp is always OFF test for control circuit continuity. If the test lamp is always ON test for a control circuit shorted to ground.

The Fuel Tank Pressure (FTP) sensor is a pressure transducer. The Alternative Fuels Engine Control Module (AF ECM) supplies about 5 volts on the FTP sensor reference voltage circuit. The AF ECM also supplies a ground circuit to the FTP sensor. When fuel level/tank pressure is full the FTP signal voltage rises to 4.1 volts. As fuel level/tank pressure decreases to empty, the FTP signal voltage drops to around 0.5 volt. The AF ECM monitors the FTP signal circuit voltage and the Fuel Tank Temperature (FTT) sensor and determines the amount of fuel within the fuel tank.

This DTC is used to indicate a FTP sensor signal voltage that is below the normal operating range of the sensor.

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

2 This step determines if the failure condition is present.
4 This step determines if the FTP sensor is the cause of the failure condition.
5 This step determines if the 5-volt reference circuit is the cause of the failure condition.

The Fuel Tank Pressure (FTP) sensor is a pressure transducer. The Alternative Fuels Engine Control Module (AF ECM) supplies about 5 volts on the FTP sensor reference voltage circuit. The AF ECM also supplies a ground circuit to the FTP sensor. When the fuel level/tank pressure is full, the FTP signal voltage rises to 4.1 volts. As the fuel level/tank pressure decreases to empty, the FTP signal voltage drops to around 0.5 volt. The AF ECM monitors the FTP signal circuit voltage and the Fuel Tank Temperature (FTT) sensor and determines the amount of fuel within the fuel tank.

This DTC is used to indicate an FTP sensor signal voltage that is greater than the normal operating range of the sensor.

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

2 This step determines if the failure condition is present.
4 This step determines if an open sensor ground circuit condition is present. Since the FTP sensor and FTT sensor share a sensor ground, an open condition between the splice and the AF ECM would cause both signal circuits to indicate high voltage.
5 This step determines if an open or shorted signal circuit is the cause of the failure condition.
6 This step determines if a shorted 5-volt reference circuit is the cause of the failure condition.
7 This step determines if an open sensor ground circuit between the FTP connector and the circuit splice is the cause of the failure condition.

Certain engine sensors require a 5-volt reference voltage source. The PCM supplies this voltage. The individual sensor 5-volt reference voltage circuits are shared within the PCM. The AF ECM monitors the 5-volt reference voltage the PCM supplies.

The AF fuel pump relay disables the gasoline fuel pump when the engine is operating on CNG. The AF ECM turns the relay ON about 2 seconds after the engine is cranked or started on gasoline. The AF ECM monitors the voltage on the AF fuel pump relay control circuit. If the AF ECM detects an incorrect voltage on the AF fuel pump relay control circuit, DTC P1650 sets.

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

2 Listen for a click when the relay operates. Command both the ON and the OFF states. Repeat the commands as necessary.
4 This step verifies that the AF ECM is providing a ground to the AF fuel pump relay.
5 This step tests for an open in the power circuit to the AF fuel pump relay.

Engine Controls Self-Diagnostics - 6.0L - Bi-Fuel & Cng: Overview GMC Sierra 2500

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Test Description

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