OBD-I Codes List

GM 56 Quad Driver Module QDM B Circuit Fault

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

How to read it

Code 56 reads as 5 flashes, pause, 6 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 56 mean?

Quad Driver Module QDM B Circuit Fault

Common causes of 56

  • Shorted relay or solenoid on the QDM B output circuit
  • Damaged wiring or corroded connector at the ECM
  • Failed relay such as cooling fan, EGR or canister purge
  • Faulty ECM quad driver output

How to fix 56

  1. Read codes with a paperclip jumper across ALDL terminals A and B and count flashes
  2. List which QDM B devices your model uses and unplug them one at a time
  3. Inspect wiring and connectors for shorts to ground or voltage
  4. Test each relay and solenoid coil for correct resistance
  5. Replace the failed part, clear the code and retest
  6. Suspect the ECM only after all outputs test clean

Source

Qualified source review

Sources and review

Review: qualified. Read the source limits below.

  • GM and Chevrolet OBD1 Trouble Codes (ALDL Fault Code List)

    live reference page (troublecodes.net)

    Supports: code identity and GM meaning

    Fetched directly. Single publisher, so the record is qualified, not verified. Causes and check order are curator-written from the circuit the code names.

    https://www.troublecodes.net/gm/

Last updated 2026-09-19.

Report a correction for 56. Include your vehicle, the issue and a source if you have one.

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