Engine Controls - Tests W/codes - 6.5l Diesel Chevrolet Cab & Chassis K3500

INTRODUCTION

The 6.5L light duty emission Diesel Electronic Control (DEC) system, electronically controls EGR system operation, Torque Converter Clutch (TCC) engagement, fuel control and fuel injection timing.

Most engine control problems are NOT computer related, but result from mechanical breakdowns, poor electrical connections, or damaged vacuum hoses. Before condemning the computer system, carefully perform visual and mechanical inspections. Failure to perform these inspections can result in lost diagnostic time.

SELF-DIAGNOSTIC SYSTEM

The 6.5L light duty emission DEC system includes a self-diagnostic system which can determine input signal circuit malfunctions. Input signal circuits determine engine function control.

Preliminary Inspection

1. Check all vacuum hoses for correct routing, restrictions, cuts or other damage. Inspect difficult-to-see vacuum hoses beneath air cleaner assembly and other engine components.
2. Inspect all engine compartment wiring for proper connections. Also check wires for pinched or chafed spots, as well as contact with sharp edges or exhaust manifolds.
3. The preliminary inspection is very important and should be performed carefully and thoroughly, as it can often fix a problem without requiring further diagnosis.

Note. Begin all diagnosis with DIAGNOSTIC SYSTEM CHECK chart. After any DEC system repair, repeat diagnostic system check.

Diagnostic Procedure

1. Ensure all engine systems NOT related to the Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all non-DEC system problems are repaired.
2. ALWAYS begin diagnosis with diagnostic system check to determine if DEC system and PCM are working properly. See «DIAGNOSTIC SYSTEM CHECK»(/chevrolet/cab-chassis-k3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-65l-diesel__diagnostic-system-check) chart. If trouble codes, other than Diagnostic Trouble Code (DTC) 12 are displayed, determine if they are hard or intermittent trouble codes.
3. A hard code is present while working on vehicle, and problem condition persists. Hard codes will cause Malfunction Indicator Light (MIL) to come on. NOTE: SERVICE ENGINE SOON light, located on instrument cluster, is also referred to as the Malfunction Indicator Light (MIL).
4. An intermittent code does not reset itself and is NOT present while working on vehicle. Intermittent codes are often caused by loose connections. MIL will go out 10 seconds after fault goes away. For intermittent diagnostic procedures, proceed to the «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-k3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel) article in this section.

ENTERING OR EXITING DIAGNOSTIC MODE

1. With ignition on and engine off, connect a jumper wire between Data Link Connector (DLC) terminal "B" (diagnostic terminal) and terminal "A" (ground). (Scheme 164) The Diesel Electronic Control (DEC) system will enter diagnostic mode.
2. In this mode, PCM will display DTC 12 by flashing the MIL once, followed by a short pause, then 2 flashes in quick succession.
3. DTC 12 will be displayed 3 times. If no other codes are stored, DTC 12 will continue to flash until diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper from DLC.

Scheme 164

PCM LOCATION

The PCM is located behind right side of dash.

SPECIAL TOOLS (DIAGNOSTIC)

Special scan testers, plugged into the DLC, may be used to read trouble codes, and check voltages in system on serial data line. These testers can save a great deal of time. For additional information, see owner's manual included with tester.

SCAN TESTER USAGE

Note. Before connecting scan tester, check diagnostic system and ensure accurate information is received by scan tester. Perform DIAGNOSTIC SYSTEM CHECK . If vehicle does not pass diagnostic system check, information received by scan tester may be invalid.

The scan tester is a specialized tester which can diagnose on-board computer control systems by providing almost instant access to circuit voltage information without crawling under dash or hood to backprobe sensors and connectors. scan testers reduce diagnostic time by furnishing input data (voltage signals) which can be compared to specification parameters. See TYPICAL TECH-1 DATA VALUES .

Scan testers also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication output signals have been sent to devices by the PCM. They do not indicate whether devices respond properly to that signal. This must be verified at output device using a voltmeter or test light.

Note. Code 12 should always exist when DLC test terminal is grounded with key on and engine off, but it may not be indicated by all makes of scan tester.

If trouble codes are not present, a problem may still exist. Driveability-related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Out-of-calibration sensors WILL NOT set a trouble code, but WILL cause driveability problems. A scan tester is the easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See TYPICAL TECH-1 DATA VALUES .

Note. Information obtained by scan tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off and remove tester. Turn ignition on and ground DLC test terminal. If same codes are not flashed by Malfunction Indicator Light (MIL) as were indicated by scan tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.

TYPICAL TECH-1 DATA VALUES

Note. Information contained in the following table is typical of readings taken on vehicle with engine idling, upper radiator hose hot, throttle closed, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Not all devices and systems are used on all models. For additional information, see tester owner's manual.

TYPICAL TECH-1 DATA VALUES

TEST EQUIPMENT

A tachometer, test light, Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, a vacuum gauge, and jumper wires are required to test and diagnose Diesel Electronic Control (DEC) system. A scan tester may also be used to access data parameters. Tester will supply a visual reading of most inputs, and some outputs, to PCM.

TDC OFFSET PROGRAM PROCEDURE

This procedure allows PCM memory to be updated with correct TDC offset for vehicle. DTC 88 will be stored until procedure has been completed.

1. Battery must be fully charged, engine operating at idle and vehicle at operating temperature.
2. Connect Tech 1 scan tool. Select "OUTPUT TESTS" and "INJ. PUMP". Activate "TDC LEARN." NOTE: It will take PCM 20 seconds to learn TDC offset.
3. Verify "TDC OFFSET" in data list. DTC will be stored if procedure has not been done correctly.
4. If PCM fails to program TDC OFFSET, do the following: Check all PCM connection. Check Techline terminal/equipment for latest software version. Try again to program PCM. If it fails again replace PCM.

DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION

Note. Trouble codes retrieved from PCM may be either engine or transmission related. Only engine-related codes are covered in this article. For transmission-related codes, see TESTS W/CODES - ELECTRONIC TRANSMISSION article in the ENGINE PERFORMANCE section. See DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION table to determine if code is engine or transmission related.

1. The DEC system codes indicate failure of a specific sensor and/or circuit. Sensor/circuit diagnosis may indicate replacement of PCM. DTC 51 indicates PROM is either improperly installed or has failed.
2. If PCM is replaced at any point during diagnostic testing and if condition is still not corrected after replacing PCM, the following may be the cause: An incorrect PCM or PROM application may cause a malfunction, which may or may not set a code. The PCM connector may be the problem. Connector terminals may have to be removed from connector to be checked properly. PROM failure. Although the PROM rarely fails, it could be the cause of the problem. Replacement PCM may be faulty. Intermittent problem. Make a careful physical inspection of affected sensor/circuit. A shorted solenoid, coil relay, or harness may be the cause of PCM failure. Use Short Circuit Tester (J-34636) to check for short circuits.

DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION

CLEARING TROUBLE CODES

Note. To prevent PCM damage, ensure ignition switch is in OFF position when disconnecting or reconnecting power to PCM.

Trouble codes should be cleared after repairs have been completed. Also, some diagnostic charts require codes to be cleared before using diagnostic chart.

1. To clear codes, with ignition off, connect a jumper wire between Data Link Connector (DLC) terminal "B" (diagnostic terminal) and terminal "A" (ground). (Scheme 164) Turn ignition on. Fully apply brake pedal, then fully apply accelerator pedal.
2. Check MIL for DTC 12. Release brake pedal, then release accelerator pedal. Check MIL again for DTC 12. Remove jumper wire from DLC. Turn ignition off. If no hard fault codes are present (only intermittent codes exist), proceed to «TESTS W/O CODES - 6.5L DIESEL»(/chevrolet/cab-chassis-k3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wo-codes-65l-diesel) article in this section for intermittent diagnostic procedures.

DIAGNOSTIC TROUBLE CODE CHARTS

Note. The following figures are courtesy of General Motors Corp.

DIAGNOSTIC SYSTEM CHECK

The diagnostic system check is an organized approach to identifying a problem created by a control module system malfunction. This is the starting point for any driveability complaint diagnosis, as it directs technician to the next logical step in diagnosis, helps reduce diagnostic time and prevents the unnecessary replacement of good parts.

Note. Test numbers refer to numbers on diagnostic chart.

1. This tests MIL operation. With ignition on and engine off, light should be on.
2. This step will isolate of customer complaint is a MIL or driveability problem.
3. Although the control module is powered up, a symptom could exist because of a system fault.
4. Use Tech-1 to aid in diagnosis (to check if serial data is available). If a PROM error is present, the PCM may have been able to flash DTC 12 or 51, but not enable serial data.
5. Although the control module is powered up, a "CRANKS BUT WILL NOT RUN" symptom could exist because of a PCM or system fault.
6. This step will isolate if customer complaint is a MIL or driveability problem with no MIL displayed. See «DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION»(/chevrolet/cab-chassis-k3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-65l-diesel) under TROUBLE CODE IDENTIFICATION, to determine if code is valid. An invalid DTC may be the result of a faulty scan tester, PROM or PCM.
7. Comparison of actual control system data with typical scan tester data values is a quick check to determine if any parameter is not within limits. A base engine problem (i.e. advanced cam timing) may substantially alter sensor values. See «TYPICAL TECH-1 DATA VALUES»(/chevrolet/cab-chassis-k3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-65l-diesel) in introduction.

Diagnostic System Check Schematic ("C" & "K" Series). Scheme 165

Diagnostic System Check Flow Chart ("C" & "K" Series). Scheme 166

CHART A-1 - NO MIL (SERVICE ENGINE SOON LIGHT)

MIL should be on when engine is off and ignition is on. Switched battery voltage is supplied to MIL. The PCM turns light on by providing a ground path through the MIL (SERVICE ENGINE SOON) control circuit.

Note. Test numbers refer to numbers on diagnostic chart.

1. If fusible link is blown, see «WIRING DIAGRAMS»(/chevrolet/cab-chassis-k3500/1989-2000/remont/testing-diagnostics/#engine-controls-tests-wcodes-65l-diesel) at the end of this article for complete circuit.
2. Using a test light connected to 12 volts, probe each of the system ground circuits to ensure a good ground is present. See «PIN VOLTAGE CHARTS»(/chevrolet/cab-chassis-k3500/1989-2000/remont/pin-voltage-charts/#engine-controls-pin-voltage-charts) article in this section.

Diagnostic Aids

If engine functions properly, check for a burned out bulb, blown gauges fuse, or an open in the MIL (SERVICE ENGINE SOON) control circuit. This will result in no brake warning light, oil or generator lights, seat belt reminder, etc.

Chart A-1 Flow Chart ("C" & "K" Series) No MIL (Service Engine Light Soon). Scheme 167

CHART A-2 - NO SCAN DATA, MIL (SERVICE ENGINE SOON) ON AT ALL TIMES OR WILL NOT FLASH MIL

MIL should be on when engine is off and ignition is on. Switched battery voltage is supplied to MIL. The PCM turns light on by providing a ground path through the MIL (SERVICE ENGINE SOON) control circuit. With diagnostic terminal grounded, MIL should flash DTC 12, followed by any other DTC stored in memory. A steady light on indicates a short to ground in the MIL (SERVICE ENGINE SOON) control circuit or an open in the diagnostic request circuit, or no engine speed sensor signal on the diesel.

Note. Test numbers refer to numbers on diagnostic chart.

1. If there is a problem with PCM that causes scan tester not to read serial data, then the PCM should not flash DTC 12. If DTC 12 does flash, ensure scan tester is functioning properly (on another vehicle). If scan tester is functioning properly, and the serial data circuit is okay, the PROM, PCM or SIR DERM may be at fault for the NO DLC symptom.
2. If light goes off when PCM connector is disconnected, then the MIL (SERVICE ENGINE SOON) control circuit is not shorted to ground.
3. This step checks for an open in the diagnostic request circuit.
4. At this point, MIL wiring is okay. The problem is a faulty PROM. If DTC 12 does not flash, the PCM should be replaced using the original PROM. Replace PROM only after trying a PCM. A defective PROM usually is an unlikely cause of the problem.

Chart A-2 Flow Chart ("C" & "K" Series) No Scan Data, MIL (Service Eng. Soon) On All Times Or Will Not Flash. Scheme 168

CHART A-3 - ENGINE CRANKS BUT WILL NOT RUN

This chart assumes battery condition and engine cranking speed are okay, there is adequate fuel in the tank, and glow plug system is operating okay.

Note. Test numbers refer to numbers on diagnostic chart.

1. A MIL on is a basic test to determine if there is a 12 volts supply and ignition 12 volts to PCM. No DLC may be due to a PCM problem and CHART A-2 will diagnose the PCM.
2. This step will check to see if there is an inject command coming from the PCM.
3. This step will check the ground circuit.

If no problem is found in the fuel pump circuit or ignition system and the cause of a "ENGINE CRANKS BUT WILL NOT RUN" has not been found, check for: low fuel pressure, water or foreign material in the fuel system, and/or a basic engine problem.

Note. If crankshaft position sensor and optical sensor are disconnected or inoperable at the same time this will cause an "ENGINE CRANKS BUT WILL NOT RUN" condition.

Chart A-3 Schematic ("C" & "K" Series) Engine Cranks But Will Not Run. Scheme 169

Chart A-3 Flow Chart ("C" & "K" Series) Engine Cranks But Will Not Run. Scheme 170

EGR CONTROL PRESSURE/BARO SENSOR OUTPUT CHECK

An EGR control pressure/BARO sensor monitors the amount of vacuum in the EGR circuit. It senses actual vacuum in the EGR vacuum line and sends a signal back to the PCM. The signal is compared to the EGR duty cycle calculated by the PCM. On vehicles not equipped with EGR, this sensor is only used for a BARO reading.

Note. Test numbers refer to numbers on diagnostic chart.

Note. Use the same diagnostic test equipment for all measurements.

1. Checks sensor output voltage to PCM. This voltage, without engine running, represents a barometer reading to the PCM. When comparing scan tester reading to a known good vehicle, it is important to compare vehicles which use a sensor having the same color insert or having the same hot stamped number.
2. Applying 10" Hg (34 kPa) vacuum to sensor should cause voltage to change. Subtract second reading from the first. Voltage value should be greater than 1.5 volts. When applying vacuum to sensor, change in voltage should be instantaneous. A slow voltage change indicates a faulty sensor.
3. Check vacuum hose to sensor for leaking or restriction. Ensure no other vacuum devices are connected to the sensor hose. NOTE: Ensure electrical connector remains securely fastened.
4. To check for intermittent connection, disconnect sensor from bracket and twist sensor by hand. Output changes greater than .1 volt indicates a bad connector or connection. If okay, replace sensor.

EGR Control Pressure/Baro Sensor Output Check Schematic ("C" & "K" Series. Scheme 171

EGR Control Pressure/Baro Sensor Output Check Flow Chart ("C" & "K" Series). Scheme 172

DTC 13 - ENGINE SHUTOFF SOLENOID CIRCUIT FAULT

When ignition switch is in OFF position, engine shutoff solenoid is in NO FUEL position. By providing a ground path, PCM energizes the solenoid which then allows fuel to pass into the fuel pump. DTC 13 will set when

1. Ignition voltage is present on the engine shutoff solenoid control circuit when PCM is requesting engine shutoff solenoid on.

Or

1. No ignition voltage is present on the engine shutoff solenoid control circuit when PCM is has requested engine shutoff solenoid off.

PCM will default to a current and history DTC 13 will be

stored.

Note. Test numbers refer to numbers on diagnostic chart.

1. Check for open circuit from ignition switch to solenoid.
2. Check the engine shutoff solenoid control circuit for open from solenoid to PCM.

An open in either engine shutoff solenoid circuit will cause DTC 13 to set. A no start condition will also exist.

DTC 13 Schematic ("C" & "K" Series) Engine Shutoff Solenoid Circuit Fault. Scheme 173

DTC 13 Flow Chart ("C" & "K" Series) Engine Shutoff Solenoid Circuit Fault. Scheme 174

DTC 14 - ECT SENSOR CIRCUIT LOW (HIGH TEMPERATURE INDICATED)

Engine Coolant Temperature (ECT) sensor is a thermistor that controls signal voltage to PCM. When engine is cold, sensor resistance is high, and PCM will sense a high signal voltage. As engine warms, sensor resistance becomes less and voltage drops. DTC 14 will set when: engine coolant temperature is greater than 304°F (151°C) for 2 seconds.

PCM will default to a engine coolant temperature of 171°F (77°C).

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 14 is the result of a hard failure or an intermittent condition.
2. This step determines if the ECT signal circuit is shorted to ground

Check wiring harness routing for a potential short circuit to ground. After starting engine, coolant temperature should rise to about 185°F (85°C). PCM default value will flash on data screen intermittently.

Test engine coolant temperature sensor at various temperature levels to evaluate the possibility of a skewed (mis-scaled) sensor. A skewed sensor could result in poor driveability complaints.

DTC 14 Schematic ("C" & "K" Series) ECT Sensor Circuit Low (High Temperature Indicated. Scheme 175

DTC 14 Flow Chart ("C" & "K" Series) ECT Sensor Circuit Low (High Temperature Indicated. Scheme 176

DTC 15 - ECT SENSOR CIRCUIT HIGH (LOW TEMPERATURE INDICATED)

Engine Coolant Temperature (ECT) sensor is a thermistor that controls signal voltage to PCM. When engine is cold, sensor resistance is high, and PCM will sense a high signal voltage. As engine warms, sensor resistance becomes less and voltage drops. DTC 15 will set when: engine coolant temperature is less than -33°F (-36°C) and engine has been running for at least 8 minutes. PCM will default to a engine coolant temperature of 64°F (18°C) and increase idle speed.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 15 is the result of a hard failure or an intermittent condition.
2. This step determines if the ECT signal circuit is open or for a faulty PCM.

Check wiring harness routing for a potential short circuit to ground. After starting engine, coolant temperature should rise to about 185°F (85°C). PCM default value will flash on data screen intermittently. Test engine coolant temperature sensor at various temperature levels to evaluate the possibility of a skewed (mis-scaled) sensor. A skewed sensor could result in poor driveability complaints.

DTC 15 Flow Chart ("C" & "K" Series) ECT Sensor Circuit High (Low Temperature Indicated). Scheme 177

DTC 16 - VEHICLE SPEED SIGNAL BUFFER FAULT

The speed sensor circuit consists of a magnetic induction type sensor, a vehicle speed sensor buffer module and wiring. Gear teeth pressed on the output shaft induce an alternating current in the sensor. This signal is transmitted to the buffer. The buffer compensates for various axle ratios and converts the signal into a square wave for use by the speedometer, cruise control, anti-lock brakes and PCM. The buffer sends 2 different signals to the PCM. The transmission output speed signal circuit relays the transmission output speed which is used to control shift points, line pressure, TCC, DTC 24 and DTC 72. The vehicle speed signal circuit relays the vehicle speed which is used to control engine operating functions and DTC 16. When DTC 24 or 72 is set, second gear only at maximum line pressure will occur. DTC 16 will set when

1. Vehicle speed greater than 20 MPH.
2. 1000-4400 RPM.
3. Vehicle speed signal circuit is open or grounded.
4. All conditions must be met for 2 seconds.

PCM will default to no cruise control or fuel cutoff.

Note. Test numbers refer to numbers on diagnostic chart.

1. This tests for battery voltage at VSS buffer.
2. This tests for proper ground path for vehicle speed sensor signal buffer.
3. This tests for vehicle speed sensor signal buffer signal to PCM.
4. This tests for a signal from VSS buffer to PCM.

Check connections at VSS buffer and PCM. Check for DTC 24 or DTC 72.

DTC 16 Schematic ("C" & "K" Series) Vehicle Speed Signal Buffer Fault. Scheme 178

DTC 16 Flow Chart ("C" & "K" Series) Vehicle Speed Signal Buffer Fault. Scheme 179

DTC 17 - HIGH RESOLUTION CIRCUIT FAULT

The optical sensor provides a high resolution signal to the PCM by counting pulses on the sensor disk located in the injection pump. The high resolution signal is one of the most important inputs used by the PCM for fuel control and timing. DTC 17 will set when 8 pump cam reference pulses occur without an increase in high resolution counts (internal to PCM). PCM will default to backup fuel.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if a 5-volt reference is present.
2. This step checks the ground circuit.
3. This step checks if the sensor is sending a signal back to the PCM.

When PCM is in backup fuel, fast idle and poor performance problems will exist. If DTC 18 is also stored, a problem with the 5-volt reference circuit or the sensor ground circuit may exist. If air is present in the fuel system DTC 17 may set.

DTC 17 Schematic High Resolution Circuit Fault. Scheme 180

DTC 17 Flow Chart ("C" & "K" Series) High Resolution Circuit Fault. Scheme 181

DTC 18 - PUMP CAM REFERENCE PULSE ERROR

The optical sensor also provides a pump cam signal to the PCM by counting pulses on the sensor disk located in the injection pump. The pump cam reference signal is one of the most important inputs used by the PCM for timing and start of injection. DTC 18 will set when 8 cam reference pulses are missed for every crankshaft position pulse. PCM will default to backup fuel.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 18 is the result of a hard failure or an intermittent condition.
2. This step checks the 5-volt reference circuits.
3. This step checks the ground circuit.
4. This step checks if the sensor is sending a signal back to the PCM and if an open is present in the cam signal circuit.

When PCM is in backup fuel, rough idle and poor performance problems will exist. If DTC 17 is also stored, a problem with the 5-volt reference circuit or the sensor ground circuit may exist.

DTC 18 Flow Chart ("C" & "K" Series) Pump Cam Reference Pulse Error. Scheme 182

DTC 19 - CRANKSHAFT POSITION REFERENCE ERROR

The crankshaft position sensor is a hall effect type sensor that monitors crankshaft position and speed There are 4 teeth 90 degrees apart on the front of the crankshaft sprocket that induce an alternating current in the sensor which is transmitted to the PCM. DTC 19 will set when 8 crankshaft position pulses are missed for every cam reference pulse.

PCM will default to backup fuel.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 19 is the result of a hard failure or an intermittent condition.
2. This step checks the 5-volt reference circuit.
3. This step checks the ground circuit.
4. This step checks the sensor and harness wiring.

When PCM is in backup fuel, fast idle and poor performance problems will exist. Check for good connections at crankshaft position sensor and PCM.

DTC 19 Schematic ("C" & "K" Series) Crankshaft Position Reference Error. Scheme 183

DTC 19 Flow Chart ("C" & "K" Series) Crankshaft Position Reference Error. Scheme 184

DTC 21 - ACCELERATOR PEDAL POSITION (APP) NO. 1 CIRCUIT HIGH

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 21 will set when voltage is greater than 4.75 volts for 2 seconds on APP No. 1 sensor. PCM will default to the input from the APP No. 1 sensor being ignored. A current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light. The throttle will operate normally as long as there is only one sensor malfunctioning.

If 2 different APP sensors have a malfunction, the SERVICE THROTTLE SOON light will light and the PCM will limit power. If 3 APP sensors malfunction, the SERVICE THROTTLE SOON light will light and the PCM will only allow the engine to operate at idle.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 21 is the result of a hard failure or an intermittent condition.
2. This step checks the PCM and wiring.
3. This step checks for an open in the ground circuit.

Scan tester displays APP No. 1 position in volts, and should display .45-.95 volt with throttle closed and ignition on or engine at idle. Voltage should increase at a steady rate as throttle is moved toward WOT. Also, 90 percent pedal travel is acceptable for correct APP operation. Monitor APP No. 1 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about .74 volt with throttle closed, to about 3.7 volts with throttle at WOT. If ground circuit is open or APP signal circuit is shorted to voltage, DTC 21 will set.

DTC 21 Schematic ("C" & "K" Series) Accelerator Pedal Position No. 1 Circuit High. Scheme 185

DTC 21 Flow Chart ("C" & "K" Series) Accelerator Pedal Position No. 1 Circuit High. Scheme 186

DTC 22 - ACCELERATOR PEDAL POSITION (APP) NO. 1 CIRCUIT LOW

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 22 will set when voltage is less than .25 volt for 2 seconds on APP No. 1 sensor.

PCM will default to the input from the APP No. 1 sensor being ignored. A current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light. The throttle will operate normally as long as there is only one sensor malfunctioning. If 2 different APP sensors have a malfunction, the SERVICE THROTTLE SOON light will light and the PCM will limit power. If 3 APP sensors malfunction, the SERVICE THROTTLE SOON light will light and the PCM will only allow the engine to operate at idle.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 22 is the result of a hard failure or an intermittent condition.
2. This step checks the PCM and wiring.
3. This step checks for a faulty connection or sensor.
4. This step checks the ground circuit.

Scan tester displays APP No. 1 position in volts, and should display .45-.95 volt with throttle closed and ignition on or engine at idle. Voltage should increase at a steady rate as throttle is moved toward WOT. Also, 90 percent pedal travel is acceptable for correct APP operation.

Monitor APP No. 1 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about .74 volt with throttle closed, to about 3.7 volts with throttle at WOT. DTC 21 will set if circuit is open.

DTC 22 Flow Chart ("C" & "K" Series). Scheme 187

DTC 23 - ACCELERATOR PEDAL POS (APP) NO. 1 CKT RANGE FAULT

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 23 will set when the PCM has recognized a skewed (mis-scaled) sensor. The PCM compares all 3 sensors to each other and determines if there is a 6 percent difference between APP No. 1 and APP No. 2, and a 10 percent difference to APP No. 3.

PCM will default to the input from the APP No. 1 sensor being ignored. A current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light. The throttle will operate normally as long as there is only one sensor malfunctioning. If 2 different APP sensors have a malfunction, the SERVICE THROTTLE SOON light will light and the PCM will limit power. If 3 APP sensors malfunction, the SERVICE THROTTLE SOON light will light and the PCM will only allow the engine to operate at idle.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if a good 5-volt reference is present.
2. This step checks for an open in the ground circuit.

Scan tester displays APP No. 1 position in volts, and should display .45-.95 volt with throttle closed and ignition on or engine at idle. Voltage should increase at a steady rate as throttle is moved toward WOT. Also, 90 percent pedal travel is acceptable for correct APP operation. Monitor APP No. 1 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about .74 volt with throttle closed, to about 3.7 volts with throttle at WOT.

DTC 23 Flow Chart ("C" & "K" Series) Accelerator Pedal Position No. 2 Circuit High. Scheme 188

DTC 25 - ACCELERATOR PEDAL POSITION (APP) NO. 2 CIRCUIT HIGH

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 25 will set when voltage is greater than 4.75 volts for 2 seconds on APP No. 2 sensor. PCM will default to the input from the APP No. 2 sensor being ignored. A current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light. The throttle will operate normally as long as there is only one sensor malfunctioning.

If 2 different APP sensors have a malfunction, the SERVICE THROTTLE SOON light will light and the PCM will limit power. If 3 APP sensors malfunction, the SERVICE THROTTLE SOON light will light and the PCM will only allow the engine to operate at idle.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 25 is the result of a hard failure or an intermittent condition.
2. This step checks for an open in ground circuit.
3. This step checks the PCM and wiring.

Scan tester displays APP No. 2 position in volts, and should display about 4.5 volts with throttle closed and ignition on or engine at idle. Voltage should decrease at a steady rate as throttle is moved toward WOT. Also, 90 percent pedal travel is acceptable for correct APP operation. Monitor APP No. 2 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about 4.5 volts with throttle closed, to about 1.5 volts with throttle at WOT.

DTC 25 Flow Chart ("C" & "K" Series) Accelerator Pedal Position (APP) No. 2 Circuit High. Scheme 189

DTC 26 - ACCELERATOR PEDAL POSITION (APP) NO. 2 CIRCUIT LOW

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 26 will set when voltage is less than .25 volt for 2 seconds on APP No. 2 sensor. PCM will default to the input from the APP No. 2 sensor being ignored. A current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light. The throttle will operate normally as long as there is only one sensor malfunctioning. If 2 different APP sensors have a malfunction, the SERVICE THROTTLE SOON light will light and the PCM will limit power. If 3 APP sensors malfunction, the SERVICE THROTTLE SOON light will light and the PCM will only allow the engine to operate at idle.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 26 is the result of a hard failure or an intermittent condition.
2. This step checks the PCM and wiring.
3. This step checks the PCM and the sensor signal circuit.

Scan tester displays APP No. 2 position in volts, and should display about 4.5 volts with throttle closed and ignition on or engine at idle. Voltage should decrease at a steady rate as throttle is moved toward WOT. Also, 90 percent pedal travel is acceptable for correct APP operation. Monitor APP No. 2 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about 4.5 volts with throttle closed, to about 1.5 volts with throttle at WOT. If 5-volt reference circuit or APP 2 signal circuit is open or shorted to ground, DTC 26 will set.

DTC 26 Flow Chart ("C" & "K" Series) Accelerator Pedal Position (APP) No. 2 Circuit Low. Scheme 190

DTC 27 - ACCELERATOR PEDAL POS (APP) NO. 2 CKT RANGE FAULT

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 27 will set when the PCM has recognized a skewed (mis-scaled) sensor. The PCM compares all 3 sensors to each other and determines if there is a 6 percent difference between APP No. 1 and APP No. 2, and a 10 percent difference to APP No. 3.

PCM will default to the input from the APP No. 2 sensor being ignored. A current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light. The throttle will operate normally as long as there is only one sensor malfunctioning. If 2 different APP sensors have a malfunction, the SERVICE THROTTLE SOON light will light and the PCM will limit power. If 3 APP sensors malfunction, the SERVICE THROTTLE SOON light will light and the PCM will only allow the engine to operate at idle.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if a good 5-volt reference is present.
2. This step checks for an open in the ground circuit.

Scan tester displays APP No. 2 position in volts, and should display about 4.5 volts with throttle closed and ignition on or engine at idle. Voltage should decrease at a steady rate as throttle is moved toward WOT. Also, 90 percent pedal travel is acceptable for correct APP operation. Monitor APP No. 2 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about 4.5 volts with throttle closed, to about 1.5 volts with throttle at WOT.

DTC 27 Flow Chart ("C" & "K" Series) Accelerator Pedal Position (APP) No. 2 Circuit Range Fault. Scheme 191

DTC 29 - GLOW PLUG RELAY CIRCUIT FAULT

The glow plug system is used to assist in providing the heat required to begin combustion during engine starting at cold ambient temperatures. The glow plugs are heated before and during cranking, as well as initial engine operation. The PCM controls the glow plugs by sending a battery voltage signal. PCM will default to hard start. DTC 29 will set when

1. Glow plugs are commanded on and glow plug voltage is less than .8 volt.

Or

1. Glow plugs are off and glow plug voltage is greater than .8 volt.

Or

1. System voltage exceeds glow plug voltage by 2 volts. NOTE: Test numbers refer to numbers on diagnostic chart. This step determines if DTC 29 is the result of a hard failure. Scan tester may not display system voltage. It is important that the PCM is cycling voltage on and off indicating the PCM is okay. This step checks the relay feed circuit. This step checks the relay ground. This step will determine if an open is present in the glow plug relay control circuit. This step checks the relay and wiring.

If glow plug relay was stuck in ON position, check for proper operation of glow plugs. When glow plugs are commanded on by scan tester, an internal PCM timer protects the glow plug from damage by cycling them on for 3 seconds and then off for 12 seconds.

DTC 29 Schematic ("C" & "K" Series) Glow Plug Relay Circuit Fault. Scheme 192

DTC 29 Flow Chart ("C" & "K" Series) Glow Plug Relay Circuit Fault. Scheme 193

DTC 31 - EGR CNTRL PRESSURE/BARO SENSOR CKT LOW (HIGH VACUUM)

A Manifold Absolute Pressure (MAP) sensor monitors the amount of vacuum in the EGR circuit and sends a signal back to PCM. The signal is compared to EGR duty cycle calculated by PCM. DTC 31 will set when: actual EGR pressure is less than 15 kPa for 2 seconds. On "C" and "K" Series, PCM will default to shutting down the EGR system.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 31 is the result of a hard failure or an intermittent condition.
2. Attaching a jumper wire between harness terminals "B" (EGR control pressure/BARO signal circuit) to "C" (5-volt reference circuit), will determine if sensor is at fault, or if there is a problem with PCM or wiring.
3. Scan tester may not display 5 volts. It is important that the PCM recognized the voltage as more than 4 volts, indicating that the EGR control pressure/BARO signal circuit and PCM are okay.

With ignition on and engine off, the EGR pressure is equal to atmospheric pressure with the signal voltage being high. This information is used by the PCM as an indication of vehicle altitude. Comparison of this reading with a known good vehicle with the same sensor is a good way to check the accuracy of a suspect sensor. Readings should be the same plus or minus .4 volt. An intermittent open in the EGR control pressure/BARO signal circuit or the 5-volt reference circuit will cause DTC 31 to set.

DTC 31 Flow Chart ("C" & "K" Series) EGR Control Pressure/Baro Sensor Circuit Low (High Vacuum). Scheme 194

DTC 32 - EGR CIRCUIT ERROR

PCM operates the normally-opened solenoid to control the EGR valve. By providing a ground path, the PCM energizes the solenoid allowing vacuum to pass to the EGR valve. During normal operation, the PCM compares EGR duty cycle signal with EGR control pressure/BARO signal and makes adjustments in duty cycle accordingly. If there is a difference in the PCM command and what is sensed by the EGR control pressure/BARO sensor, PCM makes minor adjustments to correct. DTC 32 will set when

1. RPM is greater than 506.
2. No DTC 31 0r 33 is stored.
3. 50 kPa difference between desired EGR and EGR pressure for 25.5 seconds.

PCM will default to shut down the EGR.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 32 is the result of a hard failure or an intermittent condition.
2. This step checks vacuum at EGR vent solenoid.
3. This step checks the EGR valve.

A vacuum leak or a pinched vacuum line may cause DTC 32 to set. Check all vacuum hoses and components connected to vacuum hoses for leaks or sharp bends. Check the vacuum source to the EGR solenoid. Also check for small leak in EGR valve.

DTC 32 Schematic ("C" & "K" Series) EGR Circuit Error. Scheme 195

DTC 32 Flow Chart ("C" & "K" Series) EGR Circuit Error. Scheme 196

DTC 33 - EGR CNTRL PRESSURE/BARO SENSOR CKT HIGH (LOW VACUUM)

The Exhaust Gas Recirculation (EGR) control pressure/BARO sensor monitors the amount of vacuum in the EGR circuit and sends a signal back to PCM. The signal is compared to EGR duty cycle calculated by PCM. DTC 33 will set when

1. EGR vent is off.
2. Desired EGR is less than 60 kPa from EGR pressure.

On "C" and "K" Series, PCM will default to shutting down the EGR system.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 33 is the result of a hard failure or an intermittent condition.
2. This step simulates conditions for a DTC 31. If the PCM recognizes the change, the PCM, and the EGR control pressure/BARO signal circuit and 5-volt reference circuit are okay.

With ignition on and engine off, the manifold pressure is equal to atmospheric pressure with the signal voltage being high. This information is used by the PCM as an indication of vehicle altitude. Comparison of this reading with a known good vehicle with the same sensor is a good way to check the accuracy of a suspect sensor. Readings should be the same plus or minus .4 volt. DTC 33 will set if an open is present in the sensor ground circuit, or if the EGR control pressure/BARO signal circuit is shorted to voltage or to the 5-volt reference circuit. Ensure electrical connectors remain securely fastened. Refer to EGR CONTROL PRESSURE/BARO SENSOR OUTPUT CHECK chart for further diagnosis.

DTC 33 Flow Chart ("C" & "K" Series) EGR Control Pressure/Baro Sensor Circuit High (Low Vacuum). Scheme 197

DTC 34 - INJECTION TIMING STEPPER (ITS) MOTOR CIRCUIT FAULT

The PCM controls injection timing with the injection timing stepper motor. To increase injection timing the PCM extends the stepper motor. To retard timing the PCM retracts the stepper motor. DTC 34 will set when

1. Engine operating at steady RPM.
2. A 5 degree difference exists between measured and desired injection timing for 20.8 seconds. NOTE: Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 34 is the result of a hard failure or an intermittent condition.
2. This step checks for an open or short in the ITS coil 1 low circuit and the ITS coil 1 high circuit.
3. This step checks for an open or short in the ITS coil 2 low circuit and the ITS coil 2 high circuit.

A hard start and possible poor performance condition may exist. Measured injection timing will freeze at the point of the fault.

DTC 34 Schematic Injection Timing Stepper Motor Circuit Fault. Scheme 198

DTC 34 Flow Chart ("C" & "K" Series) Injection Timing Stepper Motor Circuit Fault. Scheme 199

DTC 35 - INJECTION PULSE WIDTH ERROR (RESPONSE TIME SHORT)

The fuel injector driver receives an inject command signal from the PCM and provides a current regulated output to the fuel solenoid that controls injection. It also returns an injection pulse width signal back to the PCM to inform it when the fuel solenoid has actually seated. This injection pulse width signal is measured in micro seconds. DTC 35 will set when

1. Battery voltage is greater than 10 volts.
2. Engine coolant temperature is greater than 68°F (20°C).
3. Injection pulse width is less than 1.5 milliseconds.

PCM will default to a fixed injection pulse width value of 1.95.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 35 is the result of a hard failure or an intermittent condition.
2. This step checks for an open in the fuel inject signal circuit.

The injection pulse width will fluctuate slightly when throttle is depressed.

DTC 35 Flow Chart ("C" & "K" Series) Injection Pulse Width Error (Response Time Short). Scheme 200

DTC 36 - INJECTION PULSE WIDTH ERROR (RESPONSE TIME LONG)

The fuel injector driver receives an inject command signal from the PCM and provides a current regulated output to the fuel solenoid that controls injection. It also returns an injection pulse width signal back to the PCM to inform it when the fuel solenoid has actually seated. This injection pulse width signal is measured in micro seconds. DTC 36 will set when

1. Battery voltage is greater than 10 volts.
2. Engine coolant temperature is greater than 68°F (20°C).
3. Injection pulse width is greater than 2.5 milliseconds. PCM will default to a fixed injection pulse width value of 1.95. NOTE: Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 36 is the result of a hard failure or an intermittent condition.
2. This step determines if the solenoid is at fault, or if the problem is with the PCM or wiring.

The injection pulse width will fluctuate slightly when throttle is depressed. A weak (mechanical failure) fuel solenoid will cause DTC 36 to set.

DTC 36 Flow Chart ("C" & "K" Series) Injection Pulse Width Error (Response Time Long). Scheme 201

DTC 41 - BRAKE SWITCH CIRCUIT FAULT

The TCC normally closed brake switch supplies a battery voltage signal on the TCC brake switch signal circuit to the PCM. The circuit is open when the brakes are applied. The brake light/cruise control normally open brake switch supplies a battery voltage signal on the cruise control/brake switch signal circuit to the PCM when the brakes are applied. DTC 41 will set when

1. Switches disagree for 10 consecutive minutes.

Or

1. TCC and cruise control brake switches are not toggling open and closed during 6 brake applications on same ignition cycle.

PCM will default to an incorrect brake signal can affect TCC, 4th gear operation, in hot mode and cruise control.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks for voltage at brake switch.
2. This step simulates brake switch closed or brakes off.
3. This step checks for ignition feed to TCC brake switch.
4. This step checks the TCC brake switch signal circuit and simulates brakes being applied.

Check customer driving habits and/or unusual traffic conditions (i.e. stop and go expressway traffic).

DTC 41 Schematic ("C" & "K" Series) Brake Switch Circuit Fault. Scheme 202

DTC 41 Flow Chart ("C" & "K" Series) Brake Switch Circuit Fault. Scheme 203

DTC 42 - FUEL TEMPERATURE CKT LOW (HIGH TEMP INDICATED)

Fuel temperature sensor is a thermistor that controls signal voltage to PCM. When fuel is cold, sensor resistance is high, and PCM will sense a high signal voltage. As fuel warms, sensor resistance becomes less and voltage drops. Fuel temperature sensor is integrated with the optical sensor. DTC 42 will set when fuel temperature is greater than 215°F (102°C) for 2 seconds. PCM will default to poor idle quality during hot conditions.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 42 is the result of a hard failure or an intermittent condition.
2. This step determines if the fuel temperature signal circuit is shorted to ground.

Scan tester reads fuel temperature in degrees centigrade. After engine is running, temperature should rise steadily. DTC 42 will set if a connection is faulty, or an open is present in the fuel temperature signal circuit or the sensor ground circuit. Test fuel temperature sensor at various temperature levels to evaluate the possibility of a skewed (mis-scaled) sensor.

DTC Flow Chart ("C" & "K" Series) Fuel Temperature Circuit Low (High Temperature Indicated). Scheme 204

DTC 43 - FUEL TEMPERATURE CKT HIGH (LOW TEMP INDICATED)

Fuel temperature sensor is a thermistor that controls signal voltage to PCM. When fuel is cold, sensor resistance is high, and PCM will sense a high signal voltage. As fuel warms, sensor resistance becomes less and voltage drops. Fuel temperature sensor is integrated with the optical sensor.

DTC 43 will set when fuel temperature is less than 6°F (-14°C) and engine has been running for at least 2 minutes. PCM will default to poor idle quality during hot conditions.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 43 is the result of a hard failure or an intermittent condition.
2. This step simulates a DTC 42. If the PCM recognizes the low signal voltage (high temperature), PCM and wiring are okay.
3. This step determines if an open is present in the fuel temperature signal circuit. 5 volts should exist at sensor connector. This will determine if a wiring problem exists or if the PCM is faulty.

Scan tester reads fuel temperature in degrees centigrade. After engine is running, temperature should rise steadily. DTC 43 will set if a connection is faulty, or an open is present in the fuel temperature signal circuit or the sensor ground circuit. Test fuel temperature sensor at various temperature levels to evaluate the possibility of a skewed (mis-scaled) sensor.

DTC 43 Flow Chart ("C" & "K" Series) Fuel Temperature Circuit High (Low Temperature Indicated). Scheme 205

DTC 44 - EGR PULSE WIDTH ERROR

PCM operates the normally-open solenoid to control the EGR valve. By providing a ground path the PCM energizes the solenoid allowing vacuum to pass to the EGR. During normal operation, the PCM compares EGR duty cycle signal with EGR control pressure/BARO signal and makes adjustments in duty cycle accordingly. If there is a difference in the PCM command and what is at the EGR valve sensed by the EGR control pressure/BARO sensor, PCM makes minor adjustments to correct. DTC 44 will set when no ignition voltage is present on the EGR control circuit when EGR solenoid is commanded on. PCM will default to shut down the EGR.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 44 is the result of a hard failure or an intermittent condition.
2. This step checks if PCM and wiring are okay.
3. This step checks the EGR solenoid.

An open in the ignition feed circuit or the EGR control circuit will cause DTC 44 to set.

DTC 44 Flow Chart EGR Pulse Width Error. Scheme 206

DTC 45 - EGR VENT ERROR

When the PCM recognizes the operating range for no EGR, the PCM energizes the EGR vent solenoid which allows rapid venting of EGR vacuum. This solenoid is normally open. DTC 45 will set when no ignition voltage is present on the EGR vent control circuit when EGR vent is commanded on. PCM will default to shut off the EGR system.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 45 is the result of a hard failure or an intermittent condition.
2. This step checks if PCM and wiring are okay.
3. This step checks the power and control circuit.

An open in the ignition feed circuit or the EGR vent control circuit will cause DTC 45 to set.

DTC 45 Flow Chart ("C" & "K" Series) EGR Vent Error. Scheme 207

DTC 46 - MIL (SERVICE ENGINE SOON LIGHT) CIRCUIT FAULT

The MIL (SERVICE ENGINE SOON) should always light when the ignition is on and the engine is not running for 2 seconds. The PCM controls the MIL and makes it light by providing a ground path. DTC 46 will set when no ignition voltage on the MIL (SERVICE ENGINE SOON light) control circuit when the PCM is requesting the MIL on. PCM will default to no MIL.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks the PCM's ability to command the MIL.
2. This step determines if an open is present in the ignition feed circuit.

Check for a faulty bulb or fuse. An open in the MIL (SERVICE ENGINE SOON) control circuit will cause DTC 46 to set.

DTC 46 Schematic ("C" & "K" Series) MIL Circuit Fault. Scheme 208

DTC 46 Flow Chart ("C" & "K" Series) MIL Circuit Fault. Scheme 209

DTC 47 - IAT SENSOR CIRCUIT LOW (HIGH TEMPERATURE INDICATED)

Intake Air Temperature (IAT) sensor is a thermistor that controls signal voltage to PCM. When the air is cold, sensor resistance is high, and PCM will sense a high signal voltage. As air warms, sensor resistance becomes less and voltage drops. DTC 47 will set when

1. Engine coolant temperature is less than 100°F (38°C).
2. Intake air temperature is greater than 205°F (96°C) for 2 seconds.

PCM will default to poor engine performance during cold operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 47 is the result of a hard failure or an intermittent condition.
2. This step determines if the IAT signal circuit is shorted to ground.

Check wiring harness routing for a potential short circuit to ground in the IAT signal circuit. Scan tester displays intake air temperature in degrees centigrade. A faulty connection or an open in the IAT signal circuit or the sensor ground circuit will cause DTC 47 to set. Test intake air temperature sensor at various temperature levels to evaluate the possibility of a skewed (mis-scaled) sensor. A skewed sensor could result in poor driveability complaints.

DTC 47 Flow Chart ("C" & "K" Series) IAT Sensor Circuit Low (High Temperature Indicated). Scheme 210

DTC 48 - IAT SENSOR CIRCUIT HIGH (LOW TEMPERATURE INDICATED)

Intake Air Temperature (IAT) sensor is a thermistor that controls signal voltage to PCM. When the air is cold, sensor resistance is high, and PCM will sense a high signal voltage. As air warms, sensor resistance becomes less and voltage drops. DTC 48 will set when intake air temperature is less than -39°F (-38°C) for 2 minutes. PCM will default to poor engine performance during cold operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 48 is the result of a hard failure or an intermittent condition.
2. This step determines if the IAT signal circuit is shorted to ground.
3. This step determines if a wiring problem is present or if PCM is faulty.

Check wiring harness routing for a potential short circuit to ground in the IAT signal circuit. Scan tester displays intake air temperature in degrees centigrade. A faulty connection or an open in the IAT signal circuit or the sensor ground circuit will cause DTC 48 to set.

Test intake air temperature sensor at various temperature levels to evaluate the possibility of a skewed (mis-scaled) sensor. A skewed sensor could result in poor driveability complaints.

DTC 48 Flow Chart IAT Sensor Circuit High (Low Temperature Indicated). Scheme 211

DTC 49 - MIL (SERVICE THROTTLE SOON LIGHT) CIRCUIT FAULT

The SERVICE THROTTLE SOON light should always light when the ignition is on and the engine is not running for 2 seconds. The PCM controls the MIL and makes it light by providing a ground path. DTC 49 will set when

1. Ignition voltage on the SERVICE THROTTLE SOON light control circuit when the PCM is requesting the SERVICE THROTTLE SOON light on.

Or

1. No ignition voltage on the SERVICE THROTTLE SOON light control circuit when the PCM is requesting the SERVICE THROTTLE SOON light off.

PCM will default to the MIL will not light, but a current and history DTC 49 will set.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks the PCM's ability to command the SERVICE THROTTLE SOON light.
2. This step determines if an open is present in the ignition feed circuit or the SERVICE THROTTLE SOON light control circuit.

Check for a faulty bulb or fuse. An open in the SERVICE THROTTLE SOON light control circuit will cause DTC 49 to set.

DTC 49 Flow Chart ("C" & "K" Series) MIL (Service Throttle Soon Light) Circuit Fault. Scheme 212

DTC 51 - PROM PROBLEM (FAULTY OR INCORRECT PROM)

Ensure all pins are fully inserted in socket. If pins are fully inserted, replace PROM, clear memory and recheck. If DTC 51 resets, replace PCM. If PCM is faulty and must be replaced, TDC OFFSET must be programmed into the new PCM. When all diagnosis and repairs are complete, clear DTC's and verify for proper operation.

DTC 54 - PCM FUEL CIRCUIT ERROR

This DTC indicates a fault in the fueling circuit of the PCM is present. Check if DTC 17 is also stored. If DTC 17 is set, refer to DTC 17 chart first. If DTC 17 is not set, replace PCM. If PCM is faulty and must be replaced, TDC OFFSET must be programmed into the new PCM. When all diagnosis and repairs are complete, clear DTC's and verify for proper operation.

DTC 56 - INJECTION PUMP CALIBRATION RESISTOR ERROR

The PCM uses a calibrated resistor mounted internally in the injection pump to determine fuel rates. The resistor value is stored in the PCM memory. If the PCM memory has been disturbed or the PCM has been replaced, the PCM will relearn the resistor value on the next ignition cycle.

DTC 56 will set when PCM has lost it's memory and is unable to read a resistor value on the next ignition cycle. Possible poor performance problem. PCM will default to the MIL will light and, a current and history DTC 56 will set. PCM will default to the lowest fuel table.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 56 is the result of a hard failure.

Check connection at fuel injector driver. Clear DTC and cycle ignition. If DTC clears, treat condition as an intermittent.

DTC 56 Flow Chart Injection Pump Calibration Resistor Error. Scheme 213

DTC 57 - PCM 5-VOLT SHORTED

The 5-volt reference is a non-varying calculated voltage. DTC 57 will set when 5-volt reference is less than one volt. PCM will default to backup fuel, no EGR and no turbo boost.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step confirms that a DTC is still present.
2. This step checks that a 5-volt reference is present from the PCM.
3. This step determines if a short to ground is present in the 5-volt reference circuits or in the PCM.

During the time the failure is present, the setting of other DTC's that share a 5-volt reference may also set.

DTC 57 Flow Chart ("C" & "K" Series) PCM 5-Volt Shorted. Scheme 214

DTC 61 - TURBO BOOST SENSOR CKT HIGH

The PCM sends a 5-volt reference signal to the boost sensor. As manifold pressure changes, the electrical resistance of the boost sensor also changes. By monitoring the sensor output voltage, PCM detects how much pressure is being produced by the turbocharger in the intake manifold. The PCM uses the boost sensor to control turbo boost and fuel at different loads. DTC 61 will set when

1. Boost signal voltage is greater than 3.9 volts.
2. RPM is less than 3500. PCM will default to no turbo boost and poor performance. NOTE: Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 61 is the result of a hard failure or an intermittent condition.
2. This step simulates conditions for DTC 62. If the PCM recognizes the change, then the PCM, and the boost signal circuit are okay.
3. This step checks for an open in the ground circuit.

With ignition on and engine not running, boost pressure is equal to atmospheric pressure with the signal voltage being high. Comparison of this reading with a known good vehicle using the same sensor is a good way to check accuracy of a suspect sensor. Readings should be the same plus or minus .4 volt. Very little boost can be attained by revving the engine in Neutral.

DTC 61 Flow Chart ("C" & "K" Series) Turbo Boost Sensor Circuit High. Scheme 215

DTC 62 - TURBO BOOST SENSOR CKT LOW

The PCM sends a 5-volt reference signal to the boost sensor. As manifold pressure changes, the electrical resistance of the boost sensor also changes. By monitoring the sensor output voltage, PCM detects how much pressure is being produced by the turbocharger in the intake manifold. The PCM uses the boost sensor to control turbo boost and fuel at different loads. DTC 62 will set when boost signal voltage is less than .8 volt. PCM will default to no turbo boost and limited fuel.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 62 is the result of a hard failure or an intermittent condition.
2. This step simulates conditions for DTC 61. If the PCM recognizes the change, then the PCM, and the 5-volt reference circuit and the boost signal circuit are okay.

With ignition on and engine not running, boost pressure is equal to atmospheric pressure with the signal voltage being high. Comparison of this reading with a known good vehicle using the same sensor is a good way to check accuracy of a suspect sensor. Readings should be the same plus or minus .4 volt. Very little boost can be attained by revving the engine in Neutral.

DTC 62 Flow Chart ("C" & "K" Series) Turbo Boost Sensor Circuit Low. Scheme 216

DTC 63 - ACCELERATOR PEDAL POSITION (APP) NO. 3 CIRCUIT HIGH

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 63 will set when voltage is greater than 4.75 volts for 2 seconds on APP No. 3 sensor.

PCM will default to the input from the APP No. 3 sensor being ignored. A current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light. The throttle will operate normally as long as there is only one sensor malfunctioning. If 2 different APP sensors have a malfunction, the SERVICE THROTTLE SOON light will light and the PCM will limit power. If 3 APP sensors malfunction, the SERVICE THROTTLE SOON light will light and the PCM will only allow the engine to operate at idle.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 63 is the result of a hard failure or an intermittent condition.
2. This step checks the PCM and wiring.
3. This step checks for an open in ground circuit and PCM.

Scan tester displays APP No. 3 position in volts, and should display 4 volts with throttle closed and ignition on or engine at idle. Voltage should decrease at a steady rate as throttle is moved toward WOT. Also, 90 percent pedal travel is acceptable for correct APP operation.

Monitor APP No. 3 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about 4 volts with throttle closed, to about 2 volts with throttle at WOT.

DTC 63 Flow Chart ("C" & "K" Series) Accelerator Pedal Position (APP) No. 3 Circuit High. Scheme 217

DTC 64 - ACCELERATOR PEDAL POSITION (APP) NO. 3 CIRCUIT LOW

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 64 will set when voltage is less than .25 volt for 2 seconds on APP No. 3 sensor.

PCM will default to the input from the APP No. 3 sensor being ignored. A current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light. The throttle will operate normally as long as there is only one sensor malfunctioning. If 2 different APP sensors have a malfunction, the SERVICE THROTTLE SOON light will light and the PCM will limit power. If 3 APP sensors malfunction, the SERVICE THROTTLE SOON light will light and the PCM will only allow the engine to operate at idle.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 64 is the result of a hard failure or an intermittent condition.
2. This step checks the PCM and wiring.
3. This step checks The PCM and the sensor signal circuit.

Scan tester displays APP No. 3 position in volts, and should display 4 volts with throttle closed and ignition on or engine at idle. Voltage should decrease at a steady rate as throttle is moved toward WOT. Also, 90 percent pedal travel is acceptable for correct APP operation.

Monitor APP No. 3 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about 4 volts with throttle closed, to about 2 volts with throttle at WOT. If the 5-volt reference circuit is open or the APP 3 signal circuit is shorted to ground, DTC 64 will set.

DTC 64 Flow Chart ("C" & "K" Series) Accelerator Pedal Position (APP) No. 3 Circuit Low. Scheme 218

DTC 65 - APP NO. 3 CIRCUIT RANGE FAULT

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 65 will set when the PCM has recognized a skewed (mis-scaled) sensor. The PCM compares all 3 sensors to each other and determines if there is a 6 percent difference between APP No. 1 and APP No. 2, and a 10 percent difference to APP No. 3.

PCM will default to the input from the APP No. 3 sensor being ignored. A current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light. The throttle will operate normally as long as there is only one sensor malfunctioning. If 2 different APP sensors have a malfunction, the SERVICE THROTTLE SOON light will light and the PCM will limit power. If 3 APP sensors malfunction, the SERVICE THROTTLE SOON light will light and the PCM will only allow the engine to operate at idle.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if a good 5-volt reference is present.
2. This step checks for an open in the ground circuit.

Scan tester displays APP No. 3 position in volts, and should display 4 volts with throttle closed and ignition on or engine at idle. Voltage should decrease at a steady rate as throttle is moved toward WOT. Also, 90 percent pedal travel is acceptable for correct APP operation.

Monitor APP No. 3 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about .1 volts with throttle closed, to about 2 volts with throttle at WOT.

DTC 65 Flow Chart ("C" & "K" Series) Accelerator Pedal Position (APP) No. 3 Circuit Range Fault. Scheme 219

DTC 71 - SET/COAST SWITCH FAULT

The cruise ON/OFF, SET/COAST and RESUME/ACCEL switches are inputs to the fuel control portion of the PCM. These inputs allow the PCM to control and hold a requested speed. The set/coast request signal circuit supplies ignition voltage to the PCM when set/coast is depressed. DTC 71 will set when

1. Cruise control is on.
2. Ignition voltage on the set/coast request signal circuit is present for more than 20 seconds.

PCM will default to the PCM disallowing all cruise inputs. TCC shift schedules may be affected.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if the cruise control switch is okay.
2. This step determines if the PCM or switch is at fault.

Check for the SET/COAST switch stuck in the engage position or for a short to voltage in the set/coast request signal circuit.

DTC 71 Schematic ("C" & "K" Series) Set/Coast Switch Fault. Scheme 220

DTC 71 Flow Chart ("C" & "K" Series) Set/Coast Switch Fault. Scheme 221

DTC 76 - RESUME/ACCEL SWITCH FAULT

The cruise ON/OFF, SET/COAST and RESUME/ACCEL switches are inputs to the fuel control portion of the PCM. These inputs allow the PCM to control and hold a requested speed. The resume/accel request signal circuit supplies ignition voltage to the PCM when resume/accel is depressed. DTC 76 will set when

1. Cruise control is on.
2. Ignition voltage on the resume/accel request signal circuit is present for more than 25.5 seconds.

PCM will default to the PCM disallowing all cruise inputs. TCC shift schedules may be affected.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if the resume/accel request signal circuit is shorted to voltage.
2. This step determines if the PCM or switch is at fault.

Check for the RESUME/ACCEL switch stuck in the engage position or for a short to voltage in the resume/accel request signal circuit.

DTC 76 Flow Chart ("C" & "K" Series) Resume/Accelerator Switch Fault. Scheme 222

DTC 78 - WASTEGATE SOLENOID FAULT

The PCM operates a solenoid to control boost. This solenoid is normally open. By providing a ground path the PCM energizes the solenoid which then allows vacuum to pass to the wastegate valve. During normal operation, the PCM compares it's wastegate duty cycle signal with the boost signal and makes corrections in the duty cycle accordingly. DTC 78 will set when

1. RPM greater than 1800.
2. Fuel rate greater than 20 mm.
3. Boost pressure greater than or less than desired (internal to PCM).

PCM will default to boost will shut down. Reduce maximum fuel.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks for a good vacuum source.
2. This step checks for a faulty vacuum line or pump.
3. This step checks the solenoid wiring.

A vacuum leak or a pinched vacuum line may cause DTC 78 to set. Check all vacuum lines and components connected to the hose for leaks or sharp bends. Check vacuum source. DTC 32 will set if a problem exists with the vacuum source.

DTC 78 Flow Chart ("C" & "K" Series) Wastegate Solenoid Fault. Scheme 223

DTC 84 - ACCELERATOR PEDAL POSITION (APP) CIRCUIT FAULT

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 84 will set when the PCM has recognized an intermittent APP fault and there are no other current APP faults set. PCM will default to when DTC 84 is set, a current and history DTC will set but will not turn on the SERVICE THROTTLE SOON light, and the vehicle will operate with limited power.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if DTC 84 is the result of a hard failure or an intermittent condition.

Check for faulty connections at the APP module, PCM and instrument cluster connector.

DTC 84 Flow Chart ("C" & "K" Series) Accelerator Pedal Position Circuit Fault. Scheme 224

DTC 88 - TDC OFFSET ERROR

This DTC indicates there is a pump timing problem. This DTC will set if TDC offset is greater than plus or minus 2 degrees, or the PCM has lost memory (TDC offset is stored in PCM memory). Check injection pump timing. If okay, program TDC OFFSET into PCM. When all diagnosis and repairs are complete, clear DTC's and verify operation.

DTC'S 91-98 - CYLINDER BALANCE FAULT

If any other DTC's are set, diagnose them first. DTC's 91-98 will not light the MIL, but will be stored as a current and history DTC. Check for basic engine mechanical or cylinder fuel delivery failures (low compression, faulty nozzle, fuel leak, restricted high pressure line, etc.).

1. DTC 91 indicates a possible mechanical error with cylinder No. 1.
2. DTC 92 indicates a possible mechanical error with cylinder No. 2.
3. DTC 93 indicates a possible mechanical error with cylinder No. 3.
4. DTC 94 indicates a possible mechanical error with cylinder No. 4.
5. DTC 95 indicates a possible mechanical error with cylinder No. 5.
6. DTC 96 indicates a possible mechanical error with cylinder No. 6.
7. DTC 97 indicates a possible mechanical error with cylinder No. 7.
8. DTC 98 indicates a possible mechanical error with cylinder No. 8.

DTC 99 - APP No. 2 (5-VOLT REFERENCE FAULT)

The Accelerator Pedal Position (APP) module provides a voltage signal that changes relative to accelerator position. There are 3 sensors located within the APP module that are scaled differently. DTC 99 will set when reference voltage on APP No. 2 is less than 4.8 volts for 2 seconds. PCM will default to when DTC 99 is set, PCM will light the SERVICE THROTTLE SOON light and limit power.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines there is a good 5-volt reference.

Scan tester displays APP No. 2 position in volts, and should display 4.5 volts with throttle closed and ignition on or engine at idle. Voltage should decrease at a steady rate as throttle is moved toward WOT.

Monitor APP No. 2 sensor while applying accelerator pedal with engine off and ignition on. Display should vary from about 4.5 volts with throttle closed, to about 1.5 volts with throttle at WOT. DTC 99 will set if a short to ground is present in the 5-volt reference circuit.

DTC 99 Flow Chart ("C" & "K" Series) APP No. 2 (5-Volt Reference Fault). Scheme 225

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - With M/T - 1 Of 2). Scheme 226

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - With M/T - 2 Of 2). Scheme 227

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - With 4L60-E - 1 Of 2). Scheme 228

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - With 4L60-E - 2 Of 2). Scheme 229

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - With 4L80-E - 1 Of 2). Scheme 230

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - With 4L80-E - 2 Of 2). Scheme 231

See also:
• TESTS W/O CODES - 6.5L DIESEL
• TESTS W/CODES - ELECTRONIC TRANSMISSION
• COMPUTER RELEARN PROCEDURES
• PIN VOLTAGE CHARTS
• DIAGNOSTIC SYSTEM CHECK
• TYPICAL TECH-1 DATA VALUES
• 13
• 14
• 15
• 16
• 17
• 18
• 19
• 21
• 22
• 23
• 25
• 26
• 27
• 29
• 31
• 32
• 33
• 34
• 35
• 36
• 41
• 42
• 43
• 44
• 45
• 46
• 47
• 48
• 49
• DTC 51 - PROM PROBLEM (FAULTY OR INCORRECT PROM)
• 54
• 56
• 57
• 61
• 62
• 64
• 65
• 71
• 76
• 78
• 84
• 88
• 91
• 99