What causes GM code 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 do I fix GM code 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

42 Ignition Bypass Circuit Error

Q: What does GM code 42 mean?

GM code 42. Ignition Bypass Circuit Error. Applies to 1982-1995 GM cars and C/K trucks, Camaro, Corvette.

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

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.

What does GM code 42 mean?

42. Ignition Bypass Circuit Error. This code sets when your vehicle computer detects 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