TUCoPS :: General Information :: carcodes.txt

Hacking GM cars' onboard computer


          New cars use computers to store and remember malfunc-
     tions 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 
     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
     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.

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