How to read GM OBD1 trouble codes with a jumper wire
OBD1 GM trucks store faults as two-digit codes that the Check Engine light can "count" by flashing. All you need is a piece of wire.
Step by step
- With the engine off, find the ALDL connector.
- Jumper terminal B to A (ground) with a wire or paperclip.
- Turn the key to ON, engine not running.
- Watch the Check Engine light. A code shows as flashes: e.g. 1 flash, pause, 2 flashes = code 12.
- 12 always comes first (three times): it means the computer is working. Then stored codes follow, each three times.
- Write the codes down, remove the jumper and turn the key off.
If the bulb is burned out you won't see flashes. If the light doesn't come on at key ON, check the bulb first.
Most common codes (TBI trucks)
| Code | Usual meaning |
|---|---|
| 12 | No fault: the computer is seeing reference pulses (the normal "all good" start code) |
| 13 | Oxygen sensor (O2): open circuit |
| 14 / 15 | Coolant temperature sensor: signal high / low |
| 21 / 22 | Throttle position sensor (TPS): signal high / low |
| 24 | Vehicle speed sensor (VSS) |
| 32 | EGR system |
| 33 / 34 | MAP sensor: signal high / low |
| 42 | Electronic spark timing (EST) |
| 43 | Knock control (ESC) |
| 44 | Lean mixture (lean O2) |
| 45 | Rich mixture (rich O2) |
| 51 | PROM error |
Meanings vary slightly between ECMs and years: confirm in your model's service manual.
How to clear codes
Pull the ECM fuse or disconnect the battery for about 10 seconds. Note this also erases the fuel learning; the engine may run oddly for a few minutes.
What codes don't tell you
A code only says which circuit went out of range, not why. A code 44 could be a real lean mixture, an aging O2 sensor or a vacuum leak. To see what's happening in real time (RPM, mixture, O2 sensor) you need live data: see a real case where codes showed nothing and the data did.
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