For most 1994 and back (OBD 1, 12 pin ALDL) short terminals A & B.
In 1995 GM changed to OBD 1.5. Some late 1994 models are also OBD 1.5. For these (OBD 1.5, 16 pin ALDL) short terminals 5 & 6.
Turn key on, but do not start engine. Find and Short terminals with a paper clip. The check engine light will begin to flash. The ECM (computer) will display a "code 12" by flashing the "service engine soon" light, indicating that the system is operating. A code 12 is simply one flash, followed by a brief pause, then two flashes in quick succession. This code will be flashed 3 times. If no other codes are stored, code 12 will flash until the diagnostic terminal ground is removed. After flashing code 12 3 times, the ECM will display any stored trouble codes. Each code will be flashed 3 times, then code 12 will be flashed again, indicating that the display of any stored trouble codes has been completed.
This procedure only works on OBDI vehicles up to '95. Do NOT try this procedure on a 96 or newer vehicle!!
You want to jumper pins A and B. That should be the two upper right pins as you look at the plug.
1. With power off, jumper pin A to pin B
2. Turn the ignition to ON but DO NOT start the vehicle.
3. The Service Engine Soon (SES) light should flash code 12 (1 flash, pause, 2 quick flashes)
4. After a longer pause, code 12 should repeat two more times.
5. If there are any codes stored in the ECM they will follow Code 12.
6. If no other codes have been stored then Code 12 will continue to repeat until the jumper is disconnected.
Remember all OBDI codes are two-digit. I will work on another post to this thread that has a list of codes.
CODE SERVICE CODE DEFINITION
11 (Saturn vehicles ONLY): Indicates transmission service
codes will be displayed next on the "Shift to D2" light
(1991-92 models) or "Temperature" indicator light (1993
and later models)
12 Diagnostic mode; no distributor signal to Electronic
Control Module; System PASS
13 Oxygen sensor signal fault - signal too low / open left
oxygen sensor circuit (Dual sensor models)
14 Coolant sensor or circuit fault - signal voltage low or
shorted
15 Coolant temperature sensor or circuit fault - signal
voltage high
16 Battery or alternator problem - voltage too high or low
Direct ignition system (DIS) fault line open or shorted to
ground
Ignition system fault - loss of 2X or Low Resolution
Pulse signal
Transmission speed error
17 RPM signal problem
Camshaft sensor - circuit problems
Electronic Control Module (ECM) computer circuit
problem - Pull-up resistor (Saturn)
18 Camshaft or Crankshaft sensor - circuit problems
Fuel Injector circuit is not working properly - possible
blown fuel injector fuse
19 Ignition system fault - Intermittent 7X signal or loss of
58X signal or 6X signal (Saturn)
21 Throttle position sensor (TPS) - signal voltage is high
22 Throttle position sensor (TPS) - signal voltage is low
Fuel cutoff relay circuit - open or shorted to ground
23 Manifold air temperature (MAT) sensor - signal voltage
is low or high
Throttle position sensor (TPS) error
Mixture Control (M/C) solenoid - open or short circuit
problems
Intake Air Temperature Sensor (IAT) low
Retrieving ECM Codes
24 Vehicle speed sensor (VSS) - open or short circuit
problems or park/neutral switch circuit problem
25 Manifold air temperature (MAT) sensor - signal voltage
is low or high
Vacuum switching valve circuit open or shorted to
ground
ATS sensor - signal voltage is high or low
26 Quad-Driver module or Quad-driver No. 1 error
27 2nd gear switch problem
Quad-Driver module or Quad-driver No. 2 error
28 3rd gear switch
Quad-Driver module or Quad-driver No. 3 error
(Corvette)
(Transmission) Fluid pressure switch assembly - open or
short circuit problems
29 4th gear switch
Quad-Driver module or Quad-driver No. 3 error
Secondary air injection system - circuit problems
31 Manifold absolute pressure (MAP) sensor - signal
voltage is low
Fuel injector
Park/Neutral switch - circuit problems
CAM sensor - circuit problems
Engine speed control governor malfunction (Van)
Turbocharger wastegate overboost
Wastegate electrical signal - open or shorted to ground
Purge solenoid voltage high (carburetor engines)
32 Barometric pressure (BARO) sensor circuit failure
Exhaust gas recirculation (EGR) valve diagnostic switch -
closed during engine start-up or open when EGR flow
requested by ECM
Electronic vacuum regulator valve (EVRV) error (EVRV
controls EGR vacuum)
33 Mass air flow (MAF) sensor - signal voltage or frequency
is high during engine idle
Manifold absolute pressure (MAP) sensor - signal
voltage is high during engine idle (Note: Engine misfire
or unstable idle may cause this code)
34 Mass air flow (MAF) sensor - signal voltage or frequency
is low during engine cruise
Manifold absolute pressure (MAP) sensor - signal
voltage is low during ignition on
Pressure sensor circuit - signal voltage too high or low
(carburetor engines)
35 Idle air control (IAC) system problem - can not set
desired RPM or idle speed actuator (ISA) carbureted
system problems
36 Mass air flow (MAF) sensor - burn-off circuit problem
Transmission shift problem (electronically controlled
transmissions only)
Direct ignition system (DIS) fault - loss of 24X signal or
extra or missing pulses in electronic spark timing (EST)
signal
Ignition system fault - loss of High Resolution Pulse
signal
37 Brake switch stuck"on"
38 Brake switch circuit fault
Knock sensor (KS) - open circuit problem
39 Torque converter clutch (TCC circuit fault)
Clutch switch circuit problems
Knock sensor (KS) - short circuit problem
41 Cam sensor (CAM) failure
Cylinder select error
Tach input error - no reference pulses during engine run
Electronic spark timing (EST) circuit - open or shorted
to ground during engine run
Direct ignition system (DIS) fault - bypass circuit open
or shorted to ground during engine run
Ignition system fault - loss of 1X Reference Pulse signal
Retrieving ECM Codes
42 Electronic spark timing (EST) circuit - open or shorted
Direct ignition system (DIS) fault - bypass circuit open
or shorted to ground during engine run
Fuel cutoff relay circuit - open or shorted to ground
43 Electronic spark timing (EST) circuit - low voltage
detected
Electronic spark control (ESC) - circuit problems
44 Lean exhaust indicated (Left side on dual oxygen models)
45 Rich exhaust indicated (Left side on dual oxygen
models)
46 Vehicle anti-theft system (VATS) failure
Power steering pressure switch failure
47 Circuit or component problem in ECM/PCM (communication
error)
Knock sensor module error (inside computer)
48 Misfire symptom
Mass air flow (MAF) sensor - circuit error
49 RPM is high at idle (possible vacuum leak)
50 System voltage is low
51 Computer problem; faulty, wrong or incorrectly installed
PROM circuit; or ECM/PCM failure
52 Calibration Package Chip or ECM fault or oil temperature
sensor fault/low engine temperature
(Corvette); faulty or missing PROM/Calibration Package
Chip; ECM problem
System voltage high for a long period of time
53 System voltage too high (over 17.7 volts to ECM) or
EGR system fault or alternator voltage not normal or
Vehicle Anti-Theft System fault
54 Fuel pump circuit fault or Mixture Control (M/C)
solenoid fault or Electronic Control Module (ECM)
fault/EGR solenoid #2 failure (3.8L VIN1)
Fuel pump relay
55 Electronic Control Module (ECM) fault or oxygen sensor
circuit fault or EGR solenoid #3 failure (3.8L VIN1) or
fuel lean monitor (Corvette)
56 Vacuum sensor circuit fault or quad driver "B" fault
(3.8L VIN1)
Corrosivity/add coolant
57 Boost control problem (3.8L VIN1)
58 Vehicle Anti-Theft System fault (3.8L)
Transmission Temperature Sensor (TTS) - short circuit
Transmission fluid temp high
59 Transmission Temp Sensor (TTS) - open
Transmission fluid temp low
60 Transmission not in drive
61 Oxygen sensor signal fault or port throttle system fault
or on-board cruise control fault (vent solenoid circuit)
Air Conditioner (A/C) system performance problems
62 Engine oil temperature sensor fault or gear switch
signal circuit fault or on-board cruise control fault
(vacuum solenoid circuit)
63 EGR flow problem or on-board cruise control problem
Servo Position Sensor (SPS) circuit fault or Manifold
Absolute Pressure (MAP) sensor fault or oxygen sensor
fault
64 EGR flow problem or on-board cruise control problem or
Manifold Absolute Pressure (MAP) sensor fault or
oxygen sensor fault
65 EGR flow problem or on-board cruise control problem
Servo Position Sensor (SPS) circuit fault or Manifold
Absolute Pressure (MAP) sensor fault or oxygen sensor
fault or fuel injector current low
66 Electronic Control Module (ECM) computer circuit fault
or air conditioning pressure sensor circuit fault or low
air conditioning refrigerant charge
(Transmission) 3-2 shift control solenoid - circuit
problems
67 Cruise control - switch circuit problems
Air Conditioner (A/C) pressure sensor - circuit problems
Torque Converter Clutch (TCC) solenoid - circuit
problems
Cruise control switches - circuit problems
Retrieving ECM Codes
68 On-board cruise control switch circuit problems Servo
Position Sensor (SPS) circuit fault or shorted A/C clutch
relay circuit (Corvette) or overdrive ratio error
69 Air conditioning head pressure switch circuit fault or air
conditioning pressure switch problem
Torque converter clutch stuck "on"
70 Air conditioning refrigerant pressure sensor circuit fault
(high pressure) or quad driver module error
71 Air conditioning evaporator temperature sensor circuit
fault (low temp.)
72 Gear select switch circuit fault - Corvette only
Vehicle Speed Sensor (VSS) - loss of signal
73 Air conditioning evaporator temperature sensor circuit
fault (high temp.)
(Transmission) Pressure control solenoid - circuit
problems
74 Traction control circuit voltage low
75 Digital EGR fault - #1 solenoid or system voltage low
(charging system problem)
Transmission voltage low
76 Digital EGR fault - #2 solenoid
77 Digital EGR fault - #3 solenoid
Primary cooling fan relay driver circuit - circuit
problems
78 Secondary cooling fan relay driver circuit - circuit
problems
79 Vehicle Speed Sensor (VSS) - signal voltage too high
Transmission Temperature Sensor (TTS) - high
temperature
80 Vehicle Speed Sensor (VSS) - signal voltage too low;
transmission component fault
81 QDM Solenoid "B" monitored voltage differs from
commanded
Anti-Lock Brake System (ABS) message fault (Saturn)
Brake switch circuit problems
82 Internal PCM communication fault (Saturn) or QDM
Solenoid "A" monitored voltage differs from commanded
Ignition system fault - 3X signal problem
83 Torque Converter Clutch (TCC) solenoid - circuit
problems
Reverse Inhibit - open or short circuit in reverse inhibit
solenoid
84 3-2 Control solenoid - open or short circuit problems
Skip shift solenoid - open or short circuit problems
85 Programmable Read Only Memory error or undefined
gear ratio (input or output sensor failure)
Torque converter clutch (TCC) - TCC is mechanically
stuck on
86 Analogue/Digital Electronic Control Module (ECM)
error or shift Solenoid "B" stuck on
87 Electrically Erasable Programmable Read Only Memory
(EEPROM) error or shift Solenoid "B" stuck off or high
gear ratio error
88 Electronic Control Module (ECM) computer circuit fault
89 Power Management fault
90 TCC error
91 Skip shift light - open or short circuit problems in skip
shift light circuit
93 Pressure control solenoid - transmission line pressure
not at desired level
95 Change oil light - wrong voltage is present in light
circuit for more than 26 seconds
96 Transmission voltage low - low system voltage possibly
caused by generator voltage supply circuit or power
train control module
Low oil light - wrong voltage is present in light circuit
for more than 26 seconds
97 Vehicle speed sensor (VSS) - output circuit problems
99 Tachometer output circuit problems
