Code 42, Electronic Spark Timing: Notes

Code 42 indicates ECM has seen an open or short to ground in High Energy Ignition Electronic Spark Timing (HEI EST) system or by-pass circuits.

1. This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
2. This tests for a normal EST ground path through ignition module. If circuit No. 423 is shorted to ground, reading will be less than 500 ohms.
3. As test light voltage touches circuit No. 424, module should switch. This causes ohmmeter to "over-range" with meter in 100-200 ohm range. A higher ohm range will indicate over 5000 ohms. This test assures module switched.
4. If module did not switch, this step tests for a short in circuit No. 423, an open in circuit No. 424 and a faulty ignition module connection or module.
5. This step confirms Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.