CODE 42 - ELECTRONIC SPARK TIMING (EST) GMC Cab & Chassis S15

CODE 42 - ELECTRONIC SPARK TIMING (EST)

Code 42 means ECM has seen an open or short to ground in the Electronic Spark Timing (EST) or by-pass circuit Nos. 423 or 424.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Confirms Code 42 and that fault causing code is 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 terminal 10, module should switch, causing ohmmeter to "overrange" (meter is in the 1000-2000 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 Nos. 423 or 424.

Code 42, Electronic Spark Timing, Schematic (S-10/15 - S & T Models). Scheme 230

Code 42, Electronic Spark Timing, Schematic (Astro - M Models). Scheme 231

Code 42, Electronic Spark Timing, Flow Chart. Scheme 232

Code 42, Electronic Spark Timing, Flow Chart. Scheme 233