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This feature applies to many GM vehicles that use 4.3-liter V6, and 5.0-liter, 5.7-liter and other V8 motors, and it is taken directly from our digital online manual 24065 for the 1988-98 Chevrolet and GMC C/K 1500/2500/3500 pickup trucks (including 1999-2000 C/K Classics), Chevy Tahoe, Blazer, and Suburban, and GMC Yukon, Yukon Denali, and Suburban, 2WD or 4WD with V6 or V8 gasoline engines.
The GMT-400 trucks and SUVs bridged the gap between the early days of computer controlled electronic fuel injection and the modern OBD-II era. The system used in the 1988-1995 was very basic and could only provide clues as to what the malfunction was. In 1996 the government mandated OBD-II system became standard in nearly every car and truck and gave a much better indication of where and what the malfunction indicated was that had cause the check engine light to light on the dashboard.
Diagnostic tool information
- A digital multimeter is necessary for checking fuel injection and emission related components (see illustration). A digital volt- ohmmeter is preferred over the older style analog multimeter for several reasons. The analog multimeter cannot display the volts-ohms or amps measurement in hundredths and thousandths increments. When working with electronic circuits which are often very low voltage, this accurate reading is most important. Another good reason for the digital multimeter is the high impedance circuit. The digital multimeter is equipped with a high resistance internal circuitry (10 million ohms). Because a voltmeter is hooked up in parallel with the circuit when testing, it is vital that none of the voltage being measured should be allowed to travel the parallel path set up by the meter itself. This dilemma does not show itself when measuring larger amounts of voltage (9 to 12 volt circuits) but if you are measuring a low voltage circuit such as the oxygen sensor signal voltage, a fraction of a volt may be a significant amount when diagnosing a problem. However, there are several exceptions where using an analog voltmeter may be necessary to test certain sensors.
Illustration 2.1 Digital multimeters can be used for testing all types of circuits; because of their high impedance, they are much more accurate than analog meters for measuring millivolts in low-voltage computer circuits
- 2 Hand-held scanners are the most powerful and versatile tools for analyzing engine management systems used on later model vehicles (see illustration) . Each brand scan tool must be examined carefully to match the year, make and model of the vehicle you are working on. Often interchangeable cartridges are available to access the particular manufacturer (Ford, GM, Chrysler, etc.). Some manufacturers will specify by continent (Asia, Europe, USA, etc.).
Illustration 2.2 Scanners like the Actron Scantool and the AutoXray XP240 are powerful diagnostic aids - programmed with comprehensive diagnostic information, they can tell you just about anything you want to know about your engine management system
- With the arrival of the Federally mandated emission control system (OBD-II), a specially designed scanner has been developed. Several tool manufacturers have released OBD-II scan tools for the home mechanic. Ask the parts salesman at a local auto parts store for additional information concerning availability and cost.
On-Board Diagnostic system general description
- The On Board Diagnostic (OBD) system consists of an Electronic Control Module (ECM) and information sensors which monitor various functions of the engine and send data back to the ECM. Note: 1996 and later OBD-II systems refer to the on-board computer as the Powertrain Control Module (PCM).
- The OBD system is analogous to the central nervous system in the human body: The sensors (nerve endings) constantly relay information to the ECM (brain), which processes the data and, if necessary, sends out a command to change the operating parameters of the engine (body).
- Here’s a specific example of how one portion of this system operates: An oxygen sensor, located in the exhaust manifold, constantly monitors the oxygen content of the exhaust gas. If the percentage of oxygen in the exhaust gas is incorrect, indicating a too rich or too lean condition, an electrical signal is sent to the ECM. The ECM takes this information, processes it and then sends a command to the fuel injection system, telling it to change the air/fuel mixture. This happens in a fraction of a second and it goes on continuously when the engine is running. The end result is an air/fuel mixture ratio which is constantly maintained at a predetermined ratio, regardless of driving conditions.
1988 through 1995 models
- One might think that a system which uses an on-board computer and electrical sensors would be difficult to diagnose. This is not necessarily the case. The Computer Command Control (CCC) system has a built-in diagnostic feature which indicates a problem by flashing a SERVICE ENGINE SOON light on the instrument panel. When this light comes on during normal vehicle operation, a fault in one of the information sensor circuits or the ECM itself has been detected. More importantly, the source of the malfunction is stored in the ECM’s memory. These early electronic fuel and emission control systems are referred to as the On-Board Diagnostic (OBD-I) systems.
- To retrieve this information from the ECM memory, you must use a short jumper wire to ground a diagnostic terminal. This terminal is part of a wiring connector known as the Assembly Line Data Link (ALDL) (see illustration 2.33) . The ALDL is located underneath the dashboard, on the left hand side.
1996 through 2000 models
- This advanced electronically controlled fuel and emissions system (OBD-II) is linked with many other related engine management systems. It consists mainly of sensors, output actuators and a Powertrain Control Module (PCM). Completing the system are various other components which respond to commands from the PCM. Note: 1996 and later OBD-II systems refer to the on-board computer as the Powertrain Control Module (PCM).
- These later models require a Scan tool for code retrieval. However, many of the information sensor checks and replacement procedures on the OBD-I system, do apply to the OBD II system. Refer to the troubleshooting tips in the beginning of this manual to gain some insight to the most likely causes to a problem.
Information sensors
- Camshaft Position (CMP) sensor – On 1996 and later models, the camshaft position sensor provides information on camshaft position. The PCM uses this information, along with the crankshaft position sensor information, to control fuel injection synchronization. The camshaft position sensor is located inside the distributor.
- Crankshaft Position (CKP) sensor – On 1996 and later models, the crankshaft position sensor senses crankshaft position (TDC) during each engine revolution. The PCM uses this information to control ignition timing and fuel injection synchronization.
- Engine Coolant Temperature (ECT) sensor - The engine coolant temperature sensor senses engine coolant temperature. The ECM/PCM uses this information to control fuel injection duration and ignition timing.
- Intake Air Temperature (IAT) sensor - The intake air temperature senses the temperature of the air entering the intake manifold. The ECM/PCM uses this information to control fuel injection duration.
- Knock Sensor (KS) - The knock sensor is a piezoelectric element that detects the sound of engine detonation, or “pinging”. The ECM/PCM uses the input signal from the knock sensor to recognize detonation and retard spark advance to avoid engine damage.
- Manifold Absolute Pressure (MAP) sensor - The manifold absolute pressure monitors intake manifold pressure and ambient barometric pressure. The ECM/PCM uses this input signal to determine engine load and adjusts fuel injection duration accordingly.
- Mass Airflow (MAF) sensor – On 1996 and later models, the mass airflow sensor measures the amount of air passing through the sensor body and ultimately entering the engine. The PCM uses this information to control fuel delivery.
- Oxygen (O2) sensor - The oxygen sensors generate a voltage signal that varies with the varying oxygen content of the exhaust gas. The ECM/PCM uses this information to determine if the fuel system is running rich or lean and make adjustments accordingly.
- Throttle Position Sensor (TPS) - The throttle position sensor senses throttle movement and position. This signal enables the ECM/PCM to determine when the throttle is closed, in a cruise position, or wide open. The ECM/PCM uses this information to control fuel delivery and ignition timing.
- Vehicle Speed Sensor (VSS) - The vehicle speed sensor provides information to the ECM/PCM to indicate vehicle speed.
- Miscellaneous PCM inputs - In addition to the various sensors, the PCM on 1996 and later models, monitors various switches and circuits to determine vehicle operating conditions. The switches and circuits include:
- Air conditioning system
- Battery voltage
- Brake On/Off switch
- Cruise control system
- EGR valve position
- Engine oil level and pressure
- EVAP system
- Fuel level and fuel tank pressure
- Ignition switch
- Park/neutral position switch
- Sensor signal and ground circuits
- Transmission controls
Output actuators
- Air conditioning clutch relay – On 1996 and later models, the PCM controls the operation of the air conditioning compressor clutch with the air conditioning clutch relay.
- Service Engine Soon light - The ECM/PCM will illuminate the Service Engine Soon light if a malfunction in the electronic engine control system occurs.
- Cruise control module – On 1996 and later models, the cruise control system operation is controlled by the PCM.
- Engine cooling fan relay – On 1996 and later models, the engine cooling fan is controlled by the PCM according to information received from the engine coolant temperature sensor.
- Linear EGR valve – On 1996 and later models, the electronic EGR valve is controlled by the PCM. Ideal EGR flow is determined by the PCM and the EGR valve pintle position is adjusted accordingly.
- EVAP canister purge and vent valve solenoids – On 1996 and later models, the evaporative emission canister purge and vent valve solenoids are operated by the PCM to purge the fuel vapor canister and route fuel vapor to the intake manifold for combustion.
- Secondary Air Injection (AIR) pump and vacuum valve/ solenoid – On 1999 and 2000 models, the PCM operates the secondary air injection pump and opens the vacuum valve to inject fresh air into the exhaust stream, lower emission levels under certain operating conditions.
- Fuel injectors - The ECM/PCM opens the fuel injectors individually in firing order sequence. The ECM/PCM also controls the time the injector is held open ( pulse width). The pulse width of the injector (measured in milliseconds) determines the amount of fuel delivered. For more information on the fuel delivery system and the fuel injectors, including injector replacement, refer to Chapter 4 of your Haynes manual.
- Fuel pump relay - The fuel pump relay is activated by the ECM/PCM with the ignition switch in the Start or Run position. When the ignition switch is turned on, the relay is activated to supply initial line pressure to the system.
- Idle Air Control valve (IAC) - The idle air control valve controls the amount of air allowed to bypass the throttle plate when the throttle valve is closed or at idle position. The more air allowed to bypass the throttle plate, the higher the idle speed. The idle air control valve opening and the resulting idle speed is controlled by the ECM/PCM.
- Ignition control module – On 1996 and later models, the PCM controls ignition timing through the ignition control module depending on engine operation conditions. The ignition control module is mounted externally to the distributor. Refer to Chapter 5 for more information on the ignition control module.
Caution: This video is for familiarization purposes only and only applies to 1996 and newer models with OBD-II systems. Read below for specific information on your vehicle.
Obtaining trouble codes
- To retrieve this information from the PCM on 1988 through 1995 models (which are OBD I models) you must use a short jumper wire to ground a diagnostic terminal. The terminal is part of an electrical connector called the Assembly Line Data Link (ALDL) (see illustration) . The ALDL is located under the dashboard, just below the instrument panel and to the left of the center console. To use the ALDL, remove the plastic cover (if equipped). With the electrical connector exposed to view, push one end of the jumper wire into the diagnostic TEST terminal and the other end into the GROUND terminal.
Illustration 2.33 Assembly Line Data Link (ALDL) terminal identification (1995 and earlier models) - A) Ground B) Diagnostic test terminal C) A.I.R. (if used) E) Serial data F) T.C.C. (if used) G) Fuel pump (CK) H) Brake sense speed input (CK)
- Turn the ignition to the ON position. Caution: Do not start the engine with the TEST terminal grounded. The “SERVICE ENGINE SOON” light should flash Trouble Code 12, indicating that the diagnostic system is working. Code 12 will consist of one flash, followed by a short pause, then two more flashes in quick succession. After a longer pause, the code will repeat itself two more times. If no other codes have been stored, Code 12 will continue to repeat itself until the jumper wire is disconnected. If additional Trouble Codes have been stored, they will follow Code 12. Again, each Trouble Code will flash three times before moving on.
- Once the code(s) have been noted, use the Trouble Code Identification information which follows to locate the source of the fault.
Note: Whenever the battery cable is disconnected, all stored Trouble Codes in the PCM are erased. Be aware of this before you disconnect the battery.
- There are additional codes that relate only to models equipped the 4L60-E or 4L80-E electronic controlled transmission. These codes represent internal transmission problems that should be left to a dealer service department or an automotive or transmission repair shop. These codes are as follows: 37, 38, 39, 52, 53 and 58 through 87.
1996 through 2000 models
- To retrieve this information from 1996 and later models (OBD II), a SCAN tool must be connected to the diagnostic connector (see illustration) . The SCAN tool is a hand-held digital computer scanner that interfaces with the on-board computer. The SCAN tool is a very powerful tool; it not only reads the trouble codes but also displays the actual operating conditions of the sensors and actuators. SCAN tools are necessary to accurately diagnose a modern computerized fuel injected engine. SCAN tools are available from automotive parts stores and specialty tool companies.
Illustration 2.37 The diagnostic connector (arrow) is located under the instrument panel (1996 and later models)
- It should be noted that the self-diagnosis feature built into this system does not detect all possible faults. If you suspect a problem with the On Board Diagnostic (OBD-II) system, but the SERVICE ENGINE SOON light has not come on and no trouble codes have been stored, take the vehicle to a dealer service department or other qualified repair shop for diagnosis.
- Furthermore, when diagnosing an engine performance, fuel economy or exhaust emissions problem (which is not accompanied by a SERVICE ENGINE SOON light) do not automatically assume the fault lies in this system. Perform all standard troubleshooting procedures, as indicated elsewhere in this manual, before turning to the On Board Diagnostic (OBD-II) system.
- Finally, since this is an electronic system, you should have a basic knowledge of automotive electronics before attempting any diagnosis. Damage to the PCM, Programmable Read Only Memory (PROM) calibration unit or related components can easily occur if care is not exercised.
Clearing diagnostic trouble codes
- After the system has been repaired, the codes must be cleared from the ECM (OBD-I) or PCM (OBD-II) memory. The codes can be cleared by disconnecting battery power from the ECM/PCM for a minimum of thirty seconds. Battery power can be disconnected from the ECM/PCM by disconnecting the power connector ( fusible link) near the positive battery terminal (if equipped), removing the IGN fuse or by disconnecting the negative battery cable from the battery. The preferred method for clearing diagnostic codes on an OBD-II model is by using a scan tool.
Caution: On models equipped with the Theftlock audio system, be sure the lockout feature is turned off before performing any procedure which requires disconnecting the battery (see the front of the Haynes manual).
- Always clear the codes from the ECM/PCM before starting the engine after a new electronic emission control component is installed onto the engine. The ECM/PCM stores the operating parameters of each sensor. The ECM/PCM may set a trouble code if a new sensor is allowed to operate before the parameters from the old sensor have been erased.
Diagnostic trouble code identification
- The accompanying list of diagnostic trouble codes is a compilation of all the codes that may be encountered using a generic scan tool. Additional trouble codes may be obtainable with the use of the manufacturer specific scan tool. Not all codes pertain to all models and not all codes will illuminate the Service Engine Soon light when set.
Caution: To prevent damage to the ECM/PCM, the ignition switch must be OFF when disconnecting or connecting power to the ECM/PCM.
Note: Disconnecting the negative battery terminal will erase any radio preset codes that have been stored.
OBD-I trouble code chart
Note: Not all codes apply to all models. Further information is available in your Haynes manual.
| Trouble Code | Circuit or system | Probable cause |
| 12 | Diagnostic | This code will flash whenever the diagnostic terminal is grounded with the ignition turned On and the engine not running. If additional trouble codes are stored in the ECM they will appear after this code has flashed three times. |
| 13 | Oxygen sensor circuit | Check the wiring and connectors from the oxygen sensor. Replace oxygen sensor.* |
| 14 | Coolant sensor circuit | If the engine is experiencing overheating problems, the problem must be rectified (high temperature indicated) before continuing. Check all wiring and connectors associated with the sensor. Replace the coolant sensor.* |
| 15 | Coolant sensor circuit | See above. Also, check the thermostat for proper operation (low temperature indicated). |
| 16 | VSS buffer | Code 16 will set if the ECM detects a signal loss from the Vehicle Speed Sensor buffer. |
| 21 | TPS circuit (signal voltage high) | Check for sticking or misadjusted TPS. Check all wiring and connections at the TPS and at the ECM. Replace TPS*. |
| 22 | TPS circuit (signal voltage low) | See above. |
| 24 | Vehicle Speed Sensor (VSS) | A fault in this circuit should be indicated only while the vehicle is in motion. Disregard code 24 if set when drive wheels are not turning. Check connections at the ECM. |
| 32 | Exhaust Gas Recirculation (EGR) failure | Check the vacuum source and all vacuum lines. Check the system electrical connectors at the ECM and EGR valve. Replace the EGR valve or ECM as necessary.* |
| 33 | Manifold Absolute Pressure (MAP) signal voltage high | Check vacuum hose(s) from MAP sensor. Check electrical sensor or circuit connections at the ECM. Replace MAP sensor.* |
| 34 | Manifold Absolute Pressure (MAP) signal voltage low | Check vacuum hose(s) from MAP sensor. Check electrical sensor or circuit connections at the ECM. Replace MAP sensor.* |
| 35 | Idle Air Control | Code 35 will set if the ECM detects an idle speed above or below the desired idle speed. Replace the IAC.* |
| 36 | Idle speed control actuator | Code 36 will set if the ECM detects an open or short circuit in the idle speed control actuator solenoid circuit. |
| 42 | Ignition Control Circuit | Check the wiring and connectors between the ignition module and the ECM. Check the ignition module. Replace the ECM.* |
| 43 | Electronic Spark Control | ESC module is not receiving a knock signal from the knock sensor. Check the knock (ESC) system sensor or the ESC module. Have the module checked by a dealer service department. |
| 44 | Lean exhaust | Check the wiring and connectors from the oxygen sensor to the ECM. Check the ECM ground terminal. Check the fuel pressure. Replace the oxygen sensor .* |
| 45 | Rich exhaust | Check the evaporative charcoal canister and its components for the presence of fuel. Check for fuel or contaminated oil. Check the fuel pressure regulator. Check for a leaking fuel injector. Check for a sticking EGR valve. Replace the oxygen sensor.* |
| 51 | PROM (MEMCAL) | Faulty or incorrect PROM (MEMCAL). Diagnosis should be performed by a dealer service department or other qualified repair shop. |
| 53 | System voltage high | Code 53 will set if the voltage at the ECM is greater than 17.1-volts. Check the charging system. |
| 54 | Fuel pump relay low voltage | Check the fuel pump relay and circuit for shorts or damage. |
| 55 | ECM failure | Check the ECM power and ground circuits. If OK, replace the ECM.* |
Note: The following codes apply to 1994 and later models only.
| 28 | Transmission range pressure switch | Code 28 will set if the ECM detects an open in one of the transmission pressure switch circuits. |
| 37 | Brake switch | Code 37 will set if the ECM detects the brake switch stuck ON. |
| 38 | Brake switch | Code 37 will set if the ECM detects the brake switch stuck OFF. |
| 39 | Torque converter | Code 39 will set if the ECM detects the torque converter clutch solenoid stuck OFF.clutch solenoid |
| 52 | System voltage high | Code 52 will set if the ECM detects ignition feed voltage greater than 16-volts for 90 minutes. |
| 53 | System voltage high | Code 53 will set if the ECM detects ignition feed voltage greater than 19.5-volts for 2 seconds. |
| 58 | Transmission fluid temperature sensor | Code 58 will set if the ECM detects high transmission fluid temperature (above 300-degrees F). |
| 59 | Transmission fluid temperature sensor | Code 59 will set if the ECM detects low transmission fluid temperature (below -40-degrees F). |
| 66 | 3-2 shift control solenoid | Code 66 will set if the ECM detects an open or short circuit in the 3-2 solenoid circuit. |
| 67 | Torque converter clutch solenoid | Code 67 will set if the ECM detects an open or short circuit in the torque converter clutch solenoid circuit. |
| 68 | Transmission slipping | Code 68 will set if the ECM detects a difference in the engine speed and the transmission input speed. |
| 69 | Torque converter clutch solenoid | Code 69 will set if the ECM detects the torque converter clutch solenoid stuck ON. |
| 72 | Vehicle Speed Sensor | Code 72 will set if the ECM detects a transmission output speed signal loss. |
| 73 | Pressure control solenoid | Code 73 will set if the ECM detects a fault in the pressure control solenoid circuit. |
| 74 | Transmission input | Code 74 will set if the ECM detects a fault in the transmission input speed sensor circuit. speed sensor |
| 75 | System voltage low | Code 75 will set if the ECM detects low ignition feed voltage. |
| 79 | Transmission fluid temperature high | Code 79 will set if the ECM detects transmission fluid temperature greater than 295-degrees F for 30 minutes. |
| 81 | 2-3 shift solenoid | Code 81 will set if the ECM detects an open or short circuit in the 2-3 shift solenoid circuit. |
| 82 | 1-2 shift solenoid | Code 82 will set if the ECM detects an open or short circuit in the 1-2 shift solenoid circuit. |
| 83 | Torque converter clutch | Code 83 will set if the ECM detects an open or short circuit in the TCC solenoid circuit. |
| 85 | Transmission gear | Code 85 will set if the ECM detects the calculated transmission ratio different than the known ratio error ratio in any gear. The ECM uses this information to detect shift solenoid problems. |
| 86 | Transmission gear ratio error | Code 86 will set if the ECM detects the calculated transmission ratio lower than the known ratio in first or second gear. The ECM uses this information to detect shift solenoid problems |
| 87 | Transmission gear ratio error | Code 87 will set if the ECM detects the calculated transmission ratio higher than the known ratio in third or fourth gear. The ECM uses this information to detect shift solenoid problems. |
*Component replacement may not cure the problem in all cases. For this reason, you may want to seek professional advice before purchasing replacement parts.
OBD-II Trouble Codes
Note: The OBD-II trouble code chart is widely available online, and quite detailed. You can find an abrevieted version of it here: Haynes OBD-II Trouble Code Chart