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GM OBD1 Diagnostic Trouble Codes & Cures
This General Motors Corporation OBD I diagnostic page explains the codes, the most probable cause and what conditions are required to set the code. To retrieve these codes all that is needed is to ground the "B" terminal of the ALCL connector with the key on, however we do not recommend this as many free software programs allow for very indepth diagnosis. When possible we give the more commonly known OBD II component name.
This information combined with proper testing procedures, specifications, and tools like a digital multimeter can help you fix the vehicle right the first time. OBD diagnostic does not have to be difficult or inconclusive.
This is not meant to be a substitute for information in the factory service manuals or the product service training manuals.
Centech Digital Multimeter
automotive multimeter measures volts, amps, ohms, frequency, temperatures
SmartSpec Diagnostic Specifications Cd
complete obd diagnostic specifications for General Motors 1990-2003
automotive multimeter measures volts, amps, ohms, frequency, temperatures
SmartSpec Diagnostic Specifications Cd
complete obd diagnostic specifications for General Motors 1990-2003
| Trouble Code | Driveability Comment | Most Probable Cause | Conditions To Set |
| ============================================================================================================ | |||
| 12 Tach Reference Pulse | "Engine Miss" | Open or Ground in wire to ECM terminal 10 | Code will set as soon as malfunction occurs |
| ============================================================================================================ | |||
| 13 Oxygen Sensor circuit output low (O2) | "Poor performance" "Poor fuel economy" |
Open in wire circuit to ECM terminal 9 | Requires 17% throttle opening for 1 minute after exceedeing closed loop requirements to set code |
| ============================================================================================================ | |||
| 14 High Engine temperature Coolant Temperature Sensor (CTS) | "Poor performance" | Ground in wire to ECM terminal 3 Short between wires to ECM terminals 3 and 7 |
Requires 2 minutes to set code above 257 degrees F coolant |
| ============================================================================================================ | |||
| 15 Low engine temperature (CTS) | None | Open in wires to ECM terminal 3 and 7 | Requires 5 minutes below 23 degrees F to set code coolant |
| ============================================================================================================ | |||
| 21 Throttle Position Sensor (TPS) | "Poor fuel economy" | Open in wires to ECM terminals 2, 21 or 22 | Requires engine speed below 800 rpm and throttle open more than 25% for more than 10 seconds to set code |
| ============================================================================================================ | |||
| 23 Carburettor Mixture Control Solenoid | "Poor fuel econmoy" "Poor driveability" |
Open or ground in wire circuit to ECM terminal 18 | Requires the engine running to set code (82-83 Oldsmobile 5.0L) Requires ignition on to set code (all others) |
| ============================================================================================================ | |||
| 24 Vehicle Speed Sensor(VSS) | "Poor fuel economy" | Open or groound to ECM terminal H Open Park/Neutral switch ground wire Open or ground in wire to ECM terminal 16 Open in wire to VSS terminal c |
Requires engine speed between 1200 and 1800 and throttle open above 32% and vehicle speed below 5 mph for 20 seconds to set code |
| ============================================================================================================ | |||
| 32 Barometric Pressure Sensor (BARO) | "Poor engine performance (sluggish)" | Short between wires to ECM terminal 1 and 21 Ground in wires to ECM terminal 1 and 21 Open in wires to ECM terminals 21 or 22 |
Code will set as soon as malfunction occurs |
| ============================================================================================================ | |||
| 34 Vacuum Sensor (Manifold Absolute Pressure (MAP) ) | "Spark knock" "No power" "Poor fuel economy" |
Ground in wire to ECM terminal 20 Open in wires to ECM terminals 20, 21 or 22 Short in wires to ECM terminals 20 and 21 |
Requires engine vacuum below 6.5 inches or above 22 inches and coolant temperature above 158 degrees F with engine speed below 650 rpm for 10 seconds to set code |
| ============================================================================================================ | |||
| 35 Idle Speed Control motor (ISC) | :Stall at idle" "Dieseling" |
Ground in wire to ECM terminal 8 | Requires coolant temperature above 158 degrees F and throttle open more than 70% to set code |
| ============================================================================================================ | |||
| 41 Tach Reference Pulse (same as code 12) | "Engine miss" | Open or ground in wire circuit to ECM terminal 10 | Requires np tach signal (engine rpm below 450) and vacuum between 9 and 24 inches to set code |
| ============================================================================================================ | |||
| 42 Electronic Spark Timing (EST) or Ignition Bypass | "No power" | Open in wire to ECM terminal 12 Open or ground in wire to ECM terminal 11 Short in wires to HEI Module terminals R and E |
Requires engine running above 450 rpm and no EST signal to set code |
| ============================================================================================================ | |||
| 43 Electronic Spark Control (ESC) | "Spark knock" "No Power" |
Open or groound in wire to ECM terminal L Ground in wires to ESC terminal J Ground in wires to ESC terminal F |
Requires retard signal continously to set code |
| ============================================================================================================ | |||
| 44 Lean Exhaust | "Poor fuel economy" "Spark knock" "Lack of power" |
Ground in wire to ECM terminal 9 Open in wire to ECM terminal 14 |
Requires part throttle in closed loop for 2 minutes to set code |
| ============================================================================================================ | |||
| 45 Rich Exhaust | "Poor fuel economy" | High voltage to ECM terminal 9 (Oxygen sensor can not fail with high voltage output) | Requires part throttle in closed loop for 2 minutes to set code |
| ============================================================================================================ | |||
| 51 Prom incorrectly installed | see diagnostic chart | ||
| ============================================================================================================ | |||
| 52 Electronic Control Module malfunction (ECM PCM) | see diagnostic chart | ||
| ============================================================================================================ | |||
| 53 Electronic Control Module malfunction (ECM PCM) | see diagnostic chart | ||
| ============================================================================================================ | |||
| 54 Carnurettor Mixture Control solenoid | "Poor fuel economy" | Short in wire between battery + and ECM terminal 18 | Requires constant high voltage to ECM terminal 18 to set code |
| ============================================================================================================ | |||
| 55 Sensor Circuits | "Poor fuel economy" "No Poewer" "Stall at idle" "Spark knock" |
Short between battery + and ECM terminals 21 or 9 Open in wire to ECM terminal 14 |
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