When the system is running on the Ignition Module, that is, no voltage on the by-pass line, the Ignition Module grounds the EST signal. The ECM expects to see no voltage on the EST line during this condition. If it sees a voltage, it sets Code 42 and will not go into the EST mode.
When the rpm for EST is reached (about 400 rpm), and by-pass voltage applied, the EST should no longer be grounded in the ignition module so the EST voltage should be varying.
If the by-pass line is open or grounded, the ignition module will not switch to EST mode, so the EST voltage will be low and Code 42 will be set.
If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded there will be no EST signal. A Code 42 will be set.
Code 42 is set when:
The scantool does not have any ability to help diagnose a Code 42 problem.
A PROM not fully seated in the ECM can result in a Code 42.
Clear Codes. Idle engine for 1 minute or until code 42 sets. Does code 42 set?