GM 42 Ignition Bypass Circuit Error
How to read it
Code 42 reads as 4 flashes, 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 42 mean?
Ignition Bypass Circuit Error
Common causes of 42
- Open or grounded EST bypass wire
- Disconnected set timing connector left open after timing work
- Failed ignition module
- Damaged wiring near hot exhaust parts
How to fix 42
- Check that the set timing connector is reconnected after timing checks
- Inspect bypass wiring for opens and shorts to ground
- Test the ignition module per the service manual
- Verify EST signal toggles with a scan tool or timing light
- Clear the code and recheck after reconnecting the timing connector
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 42. Include your vehicle, the issue and a source if you have one.