Title:Pre-1996 ENGINE Trouble Codes

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.
 

TROUBLE CODES:
*Exp. Is for possible expanded information on the code above it, it may not apply to every vehicle.
The codes in italic are not for stock S-series and maybe applicable to those that have had engine and/or transmission swaps.

Code Probable Cause
12 Diagnostic Mode
13 Oxygen (O2) sensor or circuit
*Exp. Signal stays low ("lean") during warm engine cruise.
14 Coolant sensor or circuit
*Exp. High temperature indicated / Signal voltage is low
15 Coolant sensor or circuit
*Exp. Low temperature indicated / Signal voltage is high
16 System voltage high / ECM voltage over 17.1 volts (could be alternator) (3.8L)
*Exp. High battery voltage
16 System voltage high or low (3.8L)
16 Distributorless Ignition System (DIS) circuit (Chevrolet only)
*Exp. DIS fault - line open or shorted to ground
16 Low resolution pulse (5.7L P-code engines)
16 Transmission speed error (4L60E or 4.3L "S" & "T" series w/ manual)
17 Crank signal circuit shorted or faulty Electronic Control Module (ECM)
17 Camshaft position sensor circuit (3.4L)
*Exp. RPM signal problem
18 Crank signal circuit open or faulty Electronic Control Module (ECM)
18 Injector circuits (engine VIN code P - 5.7L)
18 Cam and crank sensor sync error (DIS engines) (3.8L)
19 Fuel pump circuit short
19 Crankshaft position sensor (1988-1991 models)
20 Fuel pump circuit open
21 Throttle Position Sensor (TPS) circuit or plunger
*Exp. Signal voltage is high during engine idle or deceleration.
22 Throttle Position Sensor (TPS) out of adjustment
*Exp. Signal voltage is low during engine idle
21/22 Grounded wide-open-throttle (WOT) circuit at same time
*Exp. Fuel cutoff relay circuit - open or shorted to ground
23 Manifold Air Temperature (MAT) sensor or circuit / low temp.
*Exp. MAT Sensor - signal voltage is low
23 Intake Air Temperature (IAT) sensor circuit (1995 models)
23 Electronic Spark Timing (EST) - bypass circuit problem (Cadillac DFI models)
24 Vehicle Speed Sensor (VSS) or circuit
*Exp. VSS - signal is constant (no pulsing) during engine deceleration.
25 Manifold Air Temperature (MAT) sensor or circuit / high temp.
*Exp. MAT Sensor - signal voltage is low
25 Modulated displacement failure (1981 Cadillac V8-6-4 only)
25 Electronic Spark timing (EST) (Cadillac HT4100 only)
25 Vacuum Switching Valve Circuit (VSVC)
*Exp. VSVC - open or shorted to ground
*Exp. ATS sensor - signal voltage is high.
26 Quad Driver Circuit/Module (dealer serviced)
26 Evaporator emission (EVAP) purge solenoid circuit (VIN P - 5.7L)
26 Throttle switch circuit shorted
27 Throttle switch circuit open
27 Gear Switch Diagnosis (dealer serviced)
*Exp. 2nd gear switch
27 Exhaust Gas Recirculation (EGR) vacuum control signal valve circuit (VIN P - 5.7L)
28 Pressure Switch Manifold check (PSM) vehicles w/ 4L80-E transmission
28 Same as Code 27 (Gear Switch Diagnosis)
*Exp. 3rd Gear Switch
27 or 28 Quad-driver error (5.7L P-code engines)
29 Same as Code 27 (Gear Switch Diagnosis)
*Exp. 4th Gear Switch
30 ISC circuit problem (Cadillac TBI)
30 RPM error (Cadillac MFI)
31 Turbo over boost (Turbo models)
31 Park/Neutral switch (3.3L)
31 Manifold Air Temperature (MAT) sensor or circuit (Cadillac DFI models)
31 Canister purge solenoid circuit
*Exp. Wastegate overboost
*Exp. Wastegate Electrical Signal open or shorted to ground.
31 Camshaft sensor or circuit
*Exp. CAM diagnostic
31 Exhaust Gas Recirculation (EGR) circuit (1988 to 1990 TBI)
31 Shorted Manifold Absolute Pressure (MAP) sensor circuit
*Exp. MAP Sensor - signal voltage is low
32 Barometric pressure (BARO) sensor or circuit (carbureted models)
*Exp. BARO Sensor circuit failure
32 Exhaust Gas Recirculation (EGR) circuit (fuel-injected models)
32 Digital Exhaust Gas Recirculation (EGR) circuit (3.1L)
*Exp. EGR valve diagnostic switch - closed during engine start-up or open when EGR flow requested by ECM or, EGR/EVRV.
32 Open Manifold Absolute Pressure (MAP) sensor circuit
33 Manifold Absolute Pressure (MAP) sensor or circuit (low vacuum)
33 Manifold Absolute Pressure (MAP) sensor signal voltage high
*Exp. MAP Sensor - signal voltage is high during engine idle. (Note: Engine mis-fire or unstable idle may cause this code.)
33 Mass Air Flow (MAF) sensor or circuit
*Exp. MAF Sensor - signal voltage or frequency is high during engine idle
34 Manifold Absolute Pressure (MAP) sensor signal voltage low
*Exp. MAP Sensor - signal voltage is low during ignition on
34 Vacuum sensor or Manifold Absolute Pressure (MAP) sensor (high vacuum) or circuit (3.8L)
34 Mass Air Flow (MAF) sensor or circuit
*Exp. MAF Sensor - signal voltage or frequency is, low during engine cruise
35 Idle Air Control (IAC) valve or circuit
*Exp. IAC system problem - can not set desired RPM.
35 Idle Speed Control (ISC) switch or circuit (shorted)
35 Barometric pressure (BARO) sensor or circuit (shorted) (Cadillac DFI models)
36 Barometric pressure (BARO) sensor or circuit (open) (Cadillac DFI models)
36 Mass Air Flow (MAF) sensor burn-off circuit
*Exp. MAF Sensor - burn-off circuit problem
36 Transaxle shift control (3.8L)
36 Closed throttle shift control (1991)
*Exp. Transmission shift problem
36 24X signal circuit error (3.4L - 1995)
36 Distributorless Ignition System (DIS) (Quad-4)
36 Distributorless Ignition System (DIS) ignition circuit (Corvette only)
*Exp. DIS fault - extra or missing pulses in Electronic Spark Timing (EST) signal
37 Manifold Air Temperature (MAT) sensor or circuit (shorted) (Cadillac HT4100)
37 Manifold Air Temperature (MAT) sensor temp. high (1984-1986)
37 Brake switch stuck on (4L60-E transmission)
38 Manifold Air Temperature (MAT) sensor or circuit (open) (Cadillac HT4100)
38 Brake Input Circuit (brake light switch)
38 Manifold Air Temperature (MAT) sensor temp. low (1984-1986)
38 Brake switch stuck off (4L60-E transmission)
*Exp. Brake switch circuit fault
39 Torque Converter Clutch (TCC)
*Exp. Clutch, Torque Converter circuits fault
39 Knock sensor circuit shorted (4.3L, manual trans., "S" & "T" pick-up)
40 Power steering pressure switch circuit
41 No distributor signals to Electronic Control Module (ECM), or faulty ignition module
41 Cam sensor or circuit (3.8L)
*Exp. Cam Sensor (CAM) failure
41 Cylinder select error
41 Quad 4 engine 1XReference (check ignition module / ECM wiring)
*Exp. Tach input Error - no reference pulses during engine run.
41 Electronic Spark Timing (EST) circuit (Cadillac)
41 Opti-spark EST circuit open or grounded (5.7L "F" and "Y" bodies)
42 Opti-spark EST circuit grounded (5.7L "F" and "Y" bodies)
42 Electronic Spark Timing (EST) circuit
*Exp. EST circuit - open or shorted to ground during engine run
42 Front oxygen (O2) sensor lean (Cadillac MFI)
*Exp. (DIS) fault - bypass circuit open or shorted to ground during engine run
*Exp. Fuel Cutoff Relay Circuit - open or shorted to ground.
43 Electronic Spark Control unit (ESC)
43 Throttle Position Sensor (TPS) out of adjustment
43 Front oxygen (O2) sensor rich (Cadillac MFI)
43 Knock sensor signal
*Exp. EST circuit - low voltage detected
44 Oxygen (O2) sensor or circuit - lean exhaust
*Exp. O2 Sensor voltage stays low after one or two minutes of engine run.
45 Oxygen (O2) sensor or circuit - rich exhaust
*Exp. O2 Sensor voltage stays high after one or two minutes of engine run
46 Power steering pressure switch (4 cylinder - air-conditioned modules)
46 Vehicle Anti-Theft System (VATS)
46 Right to left fueling imbalance (Cadillac)
47 Air conditioning clutch and cruise circuit
47 Electronic Control Module (ECM) computer circuit problems
*Exp. ECM - Universal Asynchronous Receiver/ Transmitter (UART) link.
48 Misfire diagnosis
48 Exhaust Gas Recirculation (EGR) system fault (Cadillac)
49 Vacuum Leak
51 PROM, MEM"CAL or ECM problem (3.1L)
*Exp. ECM computer circuit problems - Programmable read-only Memory (PROM) or checksum errors.
52 CALPAK or ECM problem (3.1L)
*Exp. ECM computer circuit problems - fuel calpac missing, analog to digital converter (A/D) error or quad driver module (QDM) fault
52 Oil Temperature Sensor (OTS)
*Exp. OTS- signal voltage is low.
53 System over-voltage (ECM over 17.7 volts)
53 Exhaust Gas Recirculation (EGR) system (carbureted models)
53 Distributor signal interrupt (1983 and later Cadillac HT4100)
53 Alternator voltage out of range
53 Vehicle anti-theft circuit (5.0L TBI)
53 Exhaust Gas Recirculation (EGR) fault (3.8L)
54 Fuel pump circuit (3.1L)
*Exp. Low fuel pump voltage
*Exp. Fuel pump relay
*Exp. Quad Driver Module (QDM) output failure.
54 Exhaust Gas Recirculation (EGR) fault (3.8L)
55 ECM/PCM error (except 5.7L PFI systems, Fuel Lean Monitor 5.7L "F" & "Y" bodies)
55 Oxygen (O2) sensor circuit or ECM
*Exp. ECM computer circuit problems - ECM failure, Serial Bus error, SAD error
55 Throttle Position Sensor (TPS) out of range (Cadillac)
55 Fuel lean monitor (Corvette)
*Exp. Fuel lean malfunction.
55 Exhaust Gas Recirculation (EGR) fault (3.8L)
56 Vacuum sensor circuit
*Exp. Part Throttle System Vacuum Sensor problems.
56 Quad driver B circuit (3.8L)
56 Anti-theft system (Cadillac)
58 PASS key fuel enable circuit
58 Transmission Temperature Sensor (TTS) high temp. (sensor or signal wire grounded)
58 Vehicle Anti-Theft System (VATS) problem.
59 Transmission Temperature Sensor (TTS) low temp. (sensor or signal wire open)
60 Transmission not in drive (Cadillac)
61 Oxygen (O2) sensor signal faulty
*Exp. O2 Sensor degraded
61 Cruise vent solenoid (3.8L)
*Exp. Cruise control problems - vent solenoid circuit
61 Secondary part throttle valve (Corvette)
*Exp. Part Throttle System error
61 Air conditioning system performance (5.7L)
62 Transaxle gear switch signal circuits (3.1L V6 / Quad-4 engines)
*Exp. Gear switch input diagnostics
62 Engine oil temperature sensor (5.7L)
*Exp. Oil Temperature Sensor - signal voltage is high
62 Cruise vacuum circuit (3.8L)
*Exp. Cruise Control Problems - vacuum solenoid circuit.
63 Manifold Absolute Pressure (MAP) sensor voltage high
63 Exhaust Gas Recirculation (EGR) flow check (3.8L)
*Exp. Small EGR failure
63 Right side Oxygen (O2) sensor circuit open (5.7L)
64 Exhaust Gas Recirculation (EGR) flow check (3.8L)
*Exp. Medium EGR failure
64 Manifold Absolute Pressure (MAP) sensor voltage low
64 Right side Oxygen (O2) sensor lean (dual sensor models)
65 Exhaust Gas Recirculation (EGR) flow check (3.8L)
*Exp. Large EGR failure
65 Right side Oxygen (O2) sensor rich (dual sensor models)
65 Cruise servo position sensor (3.8L)
65 Fuel Injection Circuit (Quad-4 engines)
*Exp. Injector Peak/hold Diagnostic
66 Air conditioning pressure sensor circuit
66 Electronic Control Module. (ECM) computer circuit problem
*Exp. ECM - Internal reset occurred.
67 Air conditioning pressure sensor or clutch circuit (Chevrolet)
67 Cruise switch circuit
68 Air conditioning relay circuit (Chevrolet)
68 Cruise system problem
69 Air conditioning clutch circuit (Chevrolet)
69 Air conditioning head pressure switch circuit
69 Transmission code - Torque converter stuck on (4L60-E)
70 Intermittent Throttle Position Sensor (TPS) (Cadillac)
70 AC refrigerant pressure sensor circuit (Chevrolet)
71 Intermittent Manifold Absolute Pressure (MAP) (Cadillac)
71 AC evaporator temperature sensor circuit(low temperature)
72 Gear selector switch (Chevrolet)
72 Throttle switch circuit (Cadillac)
72 Transmission code - Vehicle Speed Sensor (VSS) signal loss (4L60-E)
73 Intermittent coolant sensor (Cadillac)
73 AC evaporator pressure sensor circuit
74 Intermittent Manifold Air Temperature (MAT) (Cadillac)
74 Traction Control System (TCS) circuit voltage low (1995 F-body VIN P)
75 Intermittent speed sensor (Cadillac)
75 Exhaust Gas Recirculation (EGR) circuit (1995)
75 System voltage low (charging system problem)
75 Transmission system voltage low (1995 F-body VIN P)
79 Transmission fluid temperature high (4L60-E)
80 Fuel system rich (Cadillac)
80 TPS idle learn (Cadillac 4.6L)
80 Transmission slipping (F-body 3.4L)
81 Cam reference problem (Cadillac)
81 Transmission code - QDM solenoid "A" (1st and 2nd gear) current error
82 Reference signal high (Cadillac)
82 Transmission code - QDM solenoid "B" (2nd and 3rd gear) current error
83 Transmission code - QDM torque converter circuit fault (4L80-E)
83 Reverse Inhibit System (F-body, manual transmission, 5.7L)
85 Idle throttle angle high (Cadillac 4.6L)
85 Throttle body service required (Cadillac)
85 Undefined gear ratio (4L80-E)
85 PROM error (1995 3.4L)
86 Transmission code - Low gear ratio (4L80-E)
87 Transmission code - High gear ratio (4L80-E)
87 EPROM error (1995 3.4L)
88 Electronic Control Module (ECM) computer circuit problem
*Exp. ECM - Internal reset occurred.
90 Transmission Torque Converter Clutch (TCC) solenoid circuit (manual transmission)
91 Skip shift lamp circuit (1995 3.4L VIN P)
93 PCS circuit current error (1995 3.4L)
95 Engine stall detected (Cadillac)
96 Transmission system voltage low (1995 F-body 3.4L)
97 Vehicle Speed Sensor (VSS) output circuit (1995 F-body 3.4L)
98 Invalid PCM program (1995 F-body 3.4L)
99 Power management, cruise control system
99 TACH output circuit (1995 F-body 3.4L VIN P)
99 Invalid PCM program
107 PCM / BCM data link problem
108 PROM checksum mismatch
109 PCM memory reset (Cadillac)
110 Generator L-terminal circuit (Cadillac)
112 EPROM failure (Cadillac)
131 Knock sensor failure (Cadillac)
132 Knock sensor failure (Cadillac)
 

When the ECM sets a trouble code, the "check engine" or "service engine soon" light will come on and a trouble code will be stored in memory. If the problem is intermittent, the light will go out after 10 seconds, when the fault goes away. However, the trouble code will stay in the ECM memory until the battery voltage to the ECM is interrupted. Removing battery voltage for 10 seconds will clear all stored trouble codes. Trouble codes should always be cleared after repairs have been completed.

Trouble codes can be very vague, and diagnosis sometimes isn't that simple. Most people think that a code 32 (EGR system) means the EGR valve is bad and needs replaced. This isn't always so!!! There are many components in the EGR system that could cause the code to set OTHER than the EGR valve itself!

**In 1996 GM changed to OBD II. For these vehicles you will need a code scanner to check the trouble codes.

Compiled from several sources.
Submitted by: Tink (formerly BAJAChic), Dougofthenorth, ircmaxell
Sugestions by: Snoman002, Betterthanyou, and 92SonomaST