OBD-I Codes List

GM 29 Quad Driver Module Fourth Gear Circuit Fault

Make
GM (1982-1995)
Category
Powertrain - Computer and Auxiliary Controls
Severity
Check Engine Light

How to read it

Code 29 reads as 2 flashes, pause, 9 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 29 mean?

Quad Driver Module Fourth Gear Circuit Fault

Common causes of 29

  • Shorted fourth gear switch or TCC solenoid circuit
  • Damaged wiring along the transmission harness
  • Failed quad driver output inside the ECM
  • Worn overdrive components preventing fourth gear engagement

How to fix 29

  1. Confirm the transmission actually shifts into fourth gear on a road test
  2. Inspect the transmission connector and harness for damage
  3. Test the fourth gear switch and solenoid resistance per the manual
  4. Repair external wiring before suspecting the ECM
  5. Clear codes and verify fourth gear signal on a repeat drive

Source

Qualified source review

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.

    https://www.troublecodes.net/GM/86-88_383/

Last updated 2026-09-19.

Report a correction for 29. Include your vehicle, the issue and a source if you have one.

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