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ironmike, ASE Master tech
Category: Chevy
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Experience:  30 years ASE Master L1 GM certified
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how to fix engine code 42 on a 1992 chevy s10

Resolved Question:

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Submitted: 8 years ago.
Category: Chevy
Expert:  ironmike replied 8 years ago.


I would check the connectors at the module and at the pcm. The module was not a bad educated guess.

Electronic Spark Timing Circuit Code 42

Electronic Spark Timing Circuit

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

  1. Code 42 means the ECM has seen an open or short to ground in the EST or bypass circuits. This test confirms Code 42 and that the fault causing the code is present.
  2. Checks for a normal EST ground path through the ignition module. An EST CKT 423 shorted to ground will also read less than 500 ohms; however, this will be checked later.
  3. As the test light voltage touches CKT 424, the module should switch causing the ohmmeter to "overrange" if the meter is in the 100-200 ohm position. Selecting the 10-20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched."
  4. The module did not switch and this step checks for:
    • EST CKT 423 shorted to ground.
    • Bypass CKT 424 open.
    • Faulty ignition module connection or module.
  1. Confirms that Code 42 is a faulty ECM and not an intermittent in CKTs 423 or 424.




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