Code 42 - Electronic Spark Timing (Est)

Code 42 says that the ECM has seen:

A grounded EST circuit may not set a code unless cranked 10 seconds or longer with circuit grounded.

NOTE: The following step numbers refer to the numbers in the accompanying flow chart(s).

This test checks operation of EST. Grounding test terminal causes timing to go to a fixed value which is normally different from that obtained with EST operating. Therefore, the timing should change. Usually the change can be heard in engine RPM. If timing varies with increase in engine RPM (module advance), a problem is indicated.
This step eliminates the ECM and ECM connections from module input. By jumpering terminals "A" and "B", the distributor reference signal is fed directly into the EST line of the module. By putting voltage through the test light on harness terminal "C", the module is switched to the EST mode and vehicle should run. If engine stops, there is no EST signal reaching the module due to open or poor connections, or the module is faulty.
By removing the jumper, you are opening the EST signal, and the engine should stop.
Since the engine ran when the module was jumpered, the problem is not in the distributor (if correct HEI module is installed). The wrong HEI module can set a Code 42.

Fig 1: Code 42 - Electronic Spark Timing (EST) Flow Chart

Fig 2: Code 42 - Electronic Spark Timing (EST) Flow Chart