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Diagnostic Trouble Codes - 1995 GMC

 13  Open Oxygen Sensor Circuit
 14  Coolant Sensor Circuit Shorted
 15  Coolant Sensor Circuit Open
 16  Vehicle Speed Sensor Buffer Fault
 21  Throttle Position Sensor (Signal Voltage High)
 22  Throttle Position Sensor (Signal Voltage Low)
 24  Vehicle Speed Sensor (VSS) Circuit
 25  IAT Sensor (MAT) Shorted
 28  Transmission Range (TR) Pressure Switch Assembly Fault
 32  Exhaust Gas Recirculation System
 33  Manifold Absolute Pressure Sensor (MAP) Voltage High)
 34  Manifold Absolute Pressure Sensor (MAP) Voltage Low)
 35  Idle Air Control (IAC) System
 37  Brake Switch Stuck "On"
 38  Brake Switch Stuck "Off"
 41  1X Signal Error
 42  Ignition Control Circuit
 43  Knock Sensor (KS) Circuit
 44  Oxygen Sensor Circuit Lean Exhaust Indicated
 45  Oxygen Sensor Circuit Rich Exhaust Indicated
 51  Faulty Mem-Cal or PROM Problem
 52  System Voltage High Long
 53  System Voltage High
 54  Fuel Pump Relay
 55  PCM
 58  Transmission Temperature Sensor Circuit (Low Temp)
 59  Transmission Temperature Sensor Circuit (High Temp)
 66  3-2 Control Solenoid Circuit Fault
 67  Torque Converter Clutch Solenoid Circuit Fault
 69  TCC Stuck "On"
 72  Vehicle Speed Sensor (VSS) Circuit
 73  Pressure Control Solenoid Error
 75  System Voltage Low
 79  Transmission Fluid - Over Temperature
 81  2-3 Shift Solenoid Circuit Fault
 82  1-2 Shift Solenoid Circuit Fault
 83  TCC Pulse Width Modulated (PWM) Solenoid Circuit Fault

To extract a trouble from the computer for diagnostic purposes, the Assembly Line Communication Link (ALDL) connector is used.

Ground terminals A and B with the ignition "ON" and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A 12 consists of one flash, a short pause, then two flashes in rapid succession.

After 12 is displayed, any stored trouble s will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All s are displayed 3 times each then a 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.

ALDL Connector (Located Underdash)

The ALDL diagnostic connector is located in the passenger compartment.

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Additional Information provided courtesy of and Warranty Direct
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