LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> GMC >> 1987 >> Forward Control 7.4 W, Standard >> Repair and Diagnosis >> Engine Performance >> System >> CCC TBI Tests W/Codes >> Diagnostic Charts >> Code 42 - Electronic Spark Timing >> Test Description
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Test Description

NOTE: Test numbers refer to test numbers on diagnostic chart.
  1. Code 42 means the ECM has seen an open, or short to ground, in EST or by-pass circuit. This confirms Code 42, and confirms that fault causing the code is also present.
  2. Checks for normal EST ground path through ignition module. An EST circuit No. 423 shorted to ground will also read less than 500 ohms (will be checked later).
  3. As test light voltage touches circuit No. 424, module should switch, causing ohmmeter to "overrange" if meter is in the 100-200 ohms position. Selecting 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 circuit No. 423 shorted to ground.
    • By-pass circuit No. 424 open.
    • Faulty ignition module connection or module.
  5. Confirms that Code 42 is a faulty ECM and not an intermittent in circuit No. 423 or circuit No. 424.
scientia non olet · About LEMON Manuals