I have a 1988 chevy celebrity with 42,000 miles that is showing a service engine code 42. What could cause a code 42?

I have a 1988 chevy celebrity with 42,000 miles that is showing a service engine code 42. What could cause a code 42?


Code 42 is an Electronic Spark Timing issue.
Engine timing is controlled by the ignition module at 400 engine rpm or less.
At engine rpm above 400 the spark timing is controlled by the Engine Control Module (ECM) also known as one of the vehicles onboard computers that monitors engine sensors in order to control engine operations.

A Unit Repair manual will provide a diagnostic flow chart that, if followed, will walk you through tests of specific circuits, related to the switching of Engine Spark Timing from the ignition module to the on board computer.