CODES USED BY COMPUTERS IN GM CARS
New cars use computers to store and remember malfunctions that occur. These are displayed in the flashing sequences in the instrument cluster. A typical sequence of instructions follows: Locate the C3 diagnostic connector (also called the "ALCL"). It is usually under the dash on the drivers side. On the Ponitac Fiero, however, it is located in the console between the seats. Short the 'A' & 'B' positions of the diagnostic conector ('B' is ground), then turn the ignition key on but do not start the engine. CHECK ENGINE OR SERVICE ENGINE SOON light on dash will flash out the number 12 ( flash - pause - flash flash ) which means the self diagnostic mode is working. This will be repeated 3 times. Any trouble codes the computer (called the Electronic Control Module or ECM) has stored will then be flashed out ( for example: code 23 is ( flash flash - pause - flash flash flash ). If more then one code has been stored, they will be flashed out in order, each repeated 3 times. Look up the code in the following chart to find the faulty circuit or component.
NOTE: Do not automatically replace a component without first checking its wiring and connectors. Also, it is good practice to test the component further using a digital volt/ohm meter as described in the vehicle's service manual. Remove the A to B short and turn off the ignition. After the repairs, clear the trouble codes from the computor's memory by disconnecting the fuse marked 'ECM' for at least 10 seconds.
C3 TROUBLE CODES
12- Normal code with ignition on and engine off. Indicates no distributor reference pulse to ECM. Not stored in memory. 13- Oxygen sensor or it circuit 14- Coolant Temperature Sensor circuit shorted. 15- Coolant Temperature Sensor circuit open. 21- Throttle Position Sensor (TPS) or its circuit. 22- Throttle Position Sensor circuit voltage low due to grounded circuit or faulty adjustment of TPS. 23- Mixture Control (MC) Solenoid circuit open or grounded. 24- Vehicle Speed Sensor (VSS) or its circuit 32- Baromteric Pressure Sensor circuit voltage low. 34- Vacuum Sensor or Manifold Absolute Pressure (MAP) circuit. 35- Idle Speed Control (SC) Switch circuit shorted. 41- No distributor reference pulse to the ECM. Unlike code 12, this will be stored in the ECM. 42- Electronic Spark Timing (EST) bypass circuit, or EST circuit open or grounded. 43- Electronic Spark Control retard signal for too long a time. 44- Lean exhaust 45- Rich exhaust 51- Prom (programmed read only memory) calibration unit faulty or improperly installed in ECM. 53- Exhaust Gas Recirculation (EGR) valve vacuum sensor has received improper vacuum signal. 54- Shorted Mixture Control Solenoid circuit and/or faulty ECM. 55- Grounded Vref (terminal 21), high Oxygen Sensor circuit sensor voltage, or faulty ECM.