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Diagnostic Trouble Codes - 1990
GMC

 13  Open Oxygen Sensor Circuit
 14  Coolant Sensor Circuit Shorted
 15  Coolant Sensor Circuit Open
 21  Throttle Position Sensor (Signal Voltage High)
 22  Open Throttle Position Sensor Circuit
 24  Vehicle Speed Sensor (VSS) Circuit
 32  Exhaust Gas Recirculation System (Over 8500 GVW)
 32  Exhaust Gas Recirculation System (Under 8500 GVW)
 33  Manifold Absolute Pressure Sensor (MAP) Voltage High)
 34  Manifold Absolute Pressure Sensor (MAP) Voltage Low)
 35  Idle Air Control (IAC) System
 42  Electronic Spark Timing (EST) Circuit
 43  Electronic Spark Control (ESC) Circuit
 44  Oxygen Sensor Circuit Lean Exhaust Indicated
 45  Oxygen Sensor Circuit Rich Exhaust Indicated
 51  Faulty Mem-Cal or PROM Problem
 52  Fuel CALPAK Missing
 53  Over Voltage Error
 54  Fuel Pump Circuit (Low Voltage)
 55  Faulty ECM

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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