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  - This page is dedicated to GM OBD I engine computer codes.

- To retrieve the engine trouble codes from the OBD I, you need to jump the terminals "A" and "B" with a short jumper wire, once you have the terminals A and B connected, turn the ignition switch on, this sets the engine control computer in Diagnostic mode.
The computer will start by flashing a code 12 ( one flash, pause, two flashes ), this first code is to let you know that the engine control computer is in diagnostic mode, it will display all the stored codes right after code twelve.
Don't forget to turn the ignition off once the codes have been displayed, remove the jumper wire once you are done. Avoid running the engine with the computer in diagnostic mode to avoid damaging its electronic circuits.
REMEMBER: It is the check engine light the one that flashes the codes when the computer is in diagnostic mode.

How do I clear the codes?
To clear the codes in the OBD I system, turn the ignition switch off, disconnect the negative battery cable for at least ten seconds, then reconnect it

How do I gather the codes in the OBD II system?
To gather the engine trouble codes in the GM OBD II system you will need either a code reader or a scanner, these electronic devices have been programed to interact with the vehicle's PCM through the OBD II diagnose connector located under the dash.
The codes can be erased with the same scanner .
 

Code 12 :   Computer is in diagnostic mode

Code 13 :   Oxygen ( O2 ) sensor signal stays low ( "lean") or sensor circuit is open or left circuit in the case of dual
                     oxygen sensors

Code 14 :   Coolant temperature ( CTS ) signal voltage is low

Code 15 :   Coolant temperature ( CTS ) signal voltage is high

Code 16 :   System voltage high/ECM voltage over 17.1 volts, possible alternator/voltage regulator problem
          OR:  DIS ( Distributorless ignition) circuit fault, line open or shorted to ground
          OR:  Low resolution pulse ( Engine VIN code P - 5.7 L )
          OR:  Transmission speed error

Code 17 :   Crank signal circuit ( shorted ) or faulty PCM

Code 18 :   Camshaft or crankshaft sensor circuit problems

Code 19 :   Fuel pump circuit problem
         OR :   Crankshaft position sensor

Code 20 :   Fuel pump circuit ( open )

Code 21 :   Throttle position sensor ( TPS ) signal voltage is high during engine idle or deceleration

Code 22 :   Throttle position sensor ( TPS ) signal voltage is low during engine idle
        OR :   Fuel cutoff relay circuit open or shorted to ground

Code 23 :   Manifold air temperature circuit  low temperature

Code 24 :   Vehicle speed sensor ( VSS ) or circuit

Code 25 :   Manifold air temperature ( MAT ) high temperature

Code 26 :   Quad driver circuit, or quad-driver no. 1 error

Code 27 :   Gear switch or quad-driver no. 2 error

Code 28 :   Transmission pressure switch manifold check ( PSM ) or quad-driver no. 3 error

Code 29 :   Same as code 27

Code 30 :   RPM error ( Cadillac )

Code 31 :   Manifold absolute pressure ( MAP ) sensor signal low, turbo over boost, or cam sensor

Code 32 :   EGR circuit or open MAP sensor circuit

Code 33 :   Manifold absolute pressure ( MAP ) sensor either high or low, or MAF sensor  or circuit

Code 34 :   MAP or MAF sensor or circuit, MAP sensor voltage low

Code 35 :   Idle air control ( IAC ) system problem

Code 36 :   Mass air flow ( MAF ) sensor burn-off circuit
         OR :   DIS Distriburless Ignition system on quad 4 engines
                  - Transaxle shift control on 3.8 L. engines
                  -  DIS Ignition system on Corvette

Code 37 :   Manifold Absolute Temperature (MAT ) sensor circuit
                  - Brake switch stuck on 4L60-E transmission

Code 38 :   MAT sensor temperature low
                  - Brake switch stuck on 4L60-E transmission

Code 39 :   TCC Torque converter clutch
                  - Knock sensor circuit shorted ( 4.3 L. engine, manual transmission, S and T pick-up)

Code 40 :   Power steering pressure switch circuit

Code 41 :   NO distributor signal to ECM
                   - Cam sensor circuit on 3.8 L. engines
                   - Electronic spark timing ( EST ) circuit Cadillac
                   - Opti-spark Electronic spark timing EST circuit open or grounded on 5.7 L. engine F and Y bodies

Code 42 :   EST Electronic spark timing circuit
                   - Front oxygen sensor lean on Cadillac MFI

Code 43 :   TPS Throttle position sensor out of adjustment
                   - Knock sensor signal

Code 44 :   Oxygen sensor or circuit-lean exhaust

Code 45 :   Oxygen sensor or circuit - rich exhaust

Code 46 :   Power steering pressure switch on 4 cylinder engines
                   - VATS vehicle anti-theft system

Code 47 :   Air conditioning clutch and cruise circuit

Code 48 :   MIsfire diagnosis
                   - EGR system fault Cadillac

Code 51 :   PROM, MEM CAL or ECM problem ( 3.1 L. )

Code 52 :   CALPAK or ECM problem ( 3.1 L. )

Code 53 :   System over-voltage ( ECM over 17.7 volts)
                  - Distributor signal interrupt 1983 and later Cadillac HT4100
                  - Vehicle anti-theft circuit ( 5.0 L. TBI )
                  - EGR fault 3.8 L.

Code 54 :  Mixture control solenoid circuit shorted ( feedback carburator system )
                  - Fuel pump circuit, low voltage indicated ( 3.1 and 3.4 L )
                  - EGR fault ( 3.8 L ) 

Code 55 :  Closed throttle angle out of range or TPS incorrectly adjusted ( Cadillac )
                  - Grounded voltage reference, faulty oxygen sensor or fuel lean ( feedback carburator )
                  - ECM/PCM error, or not grounded ( except 5.7 PFI systems, fuel lean monitor 5.7 L 
                    PFI systems, fuel lean monitor 5.7 L "F" and "Y" bodies )    
                  - ECM fault ( Buick and Oldsmobile )
                  - TPS out of range or out of adjustment ( Cadillac )
                  - Fuel lean monitor ( Corvette and F body VIN P 5.7 L )
                  - EGR fault ( 3.8 L )                                   
                 
Code 56 :  Transaxle speed sensor input circuit fault ( Cadillac )
                  - Vacuum sensor circuit
                  - Quad driver no. 2 circuit ( 3.8 L )
                  - Secondary air inlet valve actuator vacuum sensor circuit signal high ( 5.7 L vin J )
                  - Anti-theft system ( Cadillac )

Code 57 :  Transaxle temperature sensor circuit shorted ( Cadillac )
                 - Boost control problem

Code 58 :  Personal automotive security system ( PASS ) control fault ( Cadillac )
                 - Vehicle anti-theft system ( PASS-Key) fuel enable circuit fault
                 - Transmission code TTS high temperature ( sensor or signal wire grounded )