       
                      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.
