How to Read GM OBD-1 DTC's

     Reading codes in any GM OBD-1 (pre-1996) vehicle is really simple to do at home, with no special tools.  Unless you consider an ordinary paper clip a special tool.  Don't be lured into purchasing a fancy "code reader" for these vehicles!

1.  The first thing you need to do (after procuring a paper clip) is to locate the vehicle's ALDL (assembly line data link) usually found under the driver's side of the dash (red arrow below).  The example below is typical of 1988-1994 Chevrolet/GMC trucks. 

2.  Locate ports "A" and "B" in the ALDL connector.  These will be the the two ports in the upper right hand corner.

3.  Make sure that the ignition key is off, and form your paper clip into a "U" shape. 

3.  Insert the ends of the paper clip into ports "A" and "B" of the ALDL connector.

4.  Turn the key to the "On" position, but DO NOT crank the engine!  Damage may result.  The check engine light will now begin a series of flashes.  It should start with a code "12" (diagnostic test active).  It will flash this code three times.  "12" will be a single flash, followed by a series of two flashes.  Then a pause, followed by a single flash, then a series of two flashes.  Then another pause, a single flash, and a series of two flashes.

Now, if any DTC's are present, they will begin to flash in the same manner.  For example, code "32" (EGR system failure) will be represented by a series of three flashes, then a series of two flashes.  Each code will repeat three times, and code "12" will repeat once all codes have been displayed.

This sequence will continue flashing until the key is returned to the off position.  Don't forget to remove your paper clip from the diagnostic port when you are finished!

GM OBD-1 Diagnostic Trouble Code List

      The following is a general list of DTC's used in pre-1996 GM vehicles.  Not all codes are supported by all models; consulting a service manual for your particular model will narrow down the following list to the ones that may pertain to you.

DTC - 12    No distributor reference pulse (diagnostic test active)

DTC - 13    Oxygen sensor circuit open or no activity

DTC - 13    Left bank O2 sensor circuit open or no activity

DTC - 14    Engine coolant temp sensor error (high temp indicated)

DTC - 15    Engine coolant temp sensor error (low temp indicated)

DTC - 16    System voltage too low

DTC - 17    Camshaft position sensor error

DTC - 21    Throttle Position sensor error (signal high)

DTC - 22    Throttle position sensor error (signal low)

DTC - 23    Intake air temp sensor error (low temp indicated)

DTC - 24    Vehicle speed sensor error (open circuit or no activity)

DTC - 25    Intake air temp sensor error (high temp indicated)

DTC - 26    Quad Driver Module (QDM A) error

DTC - 28    Auto transmission range pressure switch error

DTC - 31    Wastegate solenoid circuit error

DTC - 31    Gear Position Switch (PRNDL) error

DTC - 32    EGR system failure

DTC - 33    MAP sensor circuit error (signal high indicating low vacuum)

DTC - 34    MAP sensor circuit error (signal low indicating high vacuum)

DTC - 35    IAC problem or idle error

DTC - 36    MAF sensor error

DTC - 36    24x Crankshaft position sensor circuit error

DTC - 36    4T60-E Shifting Error

DTC - 37    TCC brake switch circuit error

DTC - 38    TCC brake switch circuit error

DTC - 39    Clutch switch circuit error

DTC - 39    TCC error

DTC - 41    Ignition control error

DTC - 41    MEM-CAL error

DTC - 41    C³I Cam Sensor Signal error

DTC - 42    Ignition bypass circuit error

DTC - 43    Knock sensor error

DTC - 44    Oxygen sensor error (lean condition indicated)

DTC - 44    Left bank O2 sensor error (lean condition indicated)

DTC - 45    Oxygen sensor error (rich condition indicated)

DTC - 45    Left bank O2 sensor error (rich condition indicated)

DTC - 46    Pass-Key II (VATS) system error - circuit out of freq range

DTC - 47    UART (serial data) circuit error

DTC - 51    PROM error

DTC - 51    Incorrect MEM-CAL

DTC - 52    Fuel cal-pak incorrect or missing

DTC - 53    System voltage too high

DTC - 53    Digital EGR Valve solenoid 1 error

DTC - 54    Fuel pump circuit low voltage

DTC - 54    Digital EGR Valve solenoid 2 error

DTC - 55    ECM error

DTC - 55    Digital EGR Valve solenoid 3 error

DTC - 56    Quad Driver Module (QDM B) error

DTC - 57    Boost Control error

DTC - 58    Trans fluid temp sensor circuit error (low voltage)

DTC - 58    VATS system error

DTC - 59    Trans fluid temp sensor circuit error (high voltage)

DTC - 61    A/C system performance

DTC - 63    Right bank O2 sensor circuit error

DTC - 63    MAP sensor circuit signal voltage high (low vacuum indicated)

DTC - 64    Right bank O2 sensor lean exhaust indicated

DTC - 64    MAP sensor circuit signal voltage low (high vacuum indicated)

DTC - 65    Right bank O2 sensor rich exhaust indicated

DTC - 66    A/C pressure sensor circuit error (low pressure indicated)

DTC - 67    A/C pressure sensor circuit error

DTC - 68    A/C clutch relay circuit error (shorted to ground)

DTC - 69    A/C clutch relay circuit error (open circuit indicated)

DTC - 69    A/C high pressure switch error

DTC - 70    A/C pressure sensor circuit error (high pressure indicated)

DTC - 71    A/C evaporator temp sensor circuit error (low temp indicated)

DTC - 72    VSS signal circuit error

DTC - 73    A/C evaporator temp sensor circuit error (high temp indicated)

DTC - 75    Digital EGR solenoid #1 circuit error

DTC - 76    Digital EGR solenoid #2 circuit error

DTC - 77    Digital EGR solenoid #3 circuit error

DTC - 79    Transmission fluid overtemp

DTC - 80    Transmission component slipping

DTC - 82    3x Crankshaft position sensor circuit error

DTC - 85    PROM error

DTC - 86    A/D error

DTC - 87    EEPROM error (flash memory error)

DTC - 90    TCC error

DTC - 93    Transmission pressure control solenoid circuit error

DTC - 96    Transmission system voltage low

DTC - 98    Invalid PCM program

DTC - 99    Invalid PCM program