Question about 1992 GMC Sonoma

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I have a 1992 gmc sonoma 2 wd 4.3L throtle body i am getting the service engine soon light and I checked the codes and I get number 42- Electronic spark timing bypass circuit or EST circuit is grounded or open. A malfunctioning HEI module can cause this code. My truck does not have HEI ignition, but it does have a electronic module in the distributor. Can this be causing the problem? Do you have any ideas what else could cause this code?

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CODE 42 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


Thank you for using fixya and keep us posted

Posted on Oct 10, 2010

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