1987 Chevy 3/4 Ton Pickup Code 42

[chika989]

Since December 2011 my '87 3/4 ton Chevy pickup has been giving Code 42. I've done a bit of online research but don't completely understand the potential fixes for this problem. Below is some info I discovered on the internet but now I don't know how to implement my new found info



Typical causes for code 42 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 have checked #3 and the PROM is properly seated in the ECM
I have checked several times #4 and connections are fine
#5 factory wiring harness looks like it has never been disturbed
#6 ignition module is new
#7 up to this point I have not replaced ECM because I don't know how to test it to see if it is faulty

#1 cause for code 42 appears to be BYPASS line is open or grounded.....could someone please explain exactly what that means and how do I check it. I don't know how to identify the BYPASS line and I don't know how to determine if it is open

#2 cause of code 42 appears to be EST line is open or grounded. sorry but again I don't know how to identify the EST line and I don't know how to determine if it is open

Any help would be much appreciated.