OBD-I Codes List

GM 12 No Distributor Reference Pulse

Make
GM (1982-1995)
Category
Powertrain - Computer and Auxiliary Controls
Severity
Check Engine Light
Applies to
1982-1995 GM cars and C/K trucks, Camaro, Corvette

How to read it

Code 12 reads as 1 flash, pause, 2 flashes on the check-engine light. Jump terminals A and B on the ALDL connector with the key off, then turn the key on with the engine off and count the check-engine flashes.

New to flash codes? The How to Read GM OBD-I Codes with a Paperclip walks through the test and what the numbers mean. The general OBD-I guide covers the rest.

What does GM code 12 mean?

No Distributor Reference Pulse

Common causes of 12

  • Normal marker flashed with key on and engine off during ALDL diagnosis
  • Engine stalled or not cranking so no reference pulses reach the ECM
  • Open or shorted distributor reference wire
  • Failed distributor pickup coil or ignition module

How to fix 12

  1. Do not replace parts for code 12 alone with the engine off since it means diagnosis mode is active
  2. Retrieve codes with a paperclip across ALDL pins A and B and count Check Engine flashes
  3. Crank the engine and check for further codes if a no start is present
  4. Test distributor reference signal and wiring only if the engine will not start
  5. Clear codes by disconnecting battery power and recheck after a road test

Sources and review

Review: qualified. Read the source limits below.

  • GM OBD1 trouble codes, model and year page

    live reference page (troublecodes.net)

    Supports: code identity and GM meaning

    Fetched directly from the linked model page. Single publisher, so the record is qualified, not verified.

Last updated 2026-09-19.

Report a correction for 12. Include your vehicle, the issue and a source if you have one.

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