Code 42


New Member
Nov 9, 2001
I got a code 42 on my GN. She stumbles and dies at this time. I changed the ignition Module and the coil pack. I also inspected wires B-4 and D-5. I looked for opens and grounds but found none. Is the ecm the last thing it could be? can anyone help? Gotta get her running before the new year.:confused:
Hey.... don't try it too long, I need to borrow that ECM too!!:D Maybe you should charge us a fee there Mick:rolleyes: See you guys later!! Happy New Year!!

Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system.

During cranking, the timing is controlled by the ignition module while the ECM monitors the engine speed. When the engine speed exceeds 400 RPM, the ECM sends a BYPASS signal to the ignition module which switches the timing to ECM control. The ECM calculates what the timing should be then "tells" the ignition module via the EST circuit.

An open or ground in the EST circuit will stall the engine and set a Code 42. The engine can be re-started but it will run on ignition module timing.

The conditions for setting this code are:

System in BYPASS mode but the ignition module is still controlling timing

- or -
Engine speed > 600 RPM with no EST pulses (ECM controlled timing) going to the ignition module for 200 msec.

Typical causes for this code include:

1) BYPASS line is open or grounded
2) EST line is open or grounded
3) PROM or CALPACK not seated properly in the ECM
4) Poor connections between ignition module and ECM
5) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise)
6) Faulty or incorrect ignition module
7) Faulty ECM
I think I had that code in my 86 GN. It turned out to be the chip. I just pulled it and put it back in. Worked fine after that.