Code 42, Electronic Spark Timing (Est)

Code 42 indicates that the ECM has seen an open or short to ground in the EST or by-pass circuits.

1. This test confirms Code 42, and that fault is present.
2. This test checks for a normal EST ground path through the ignition module. An EST circuit No. 423 shorted to ground will read less than 500 ohms.
3. As the test light voltage touches terminal "C7", the module should switch causing the ohmmeter to "overrange" if the meter is in the 1000-2000 ohm position. Selecting a higher ohms position will indicate above 5000 ohms. It is only important that the module "switched".
4. If the module did not switch, this step will check for EST circuit No. 423 shorted to ground, an open by-pass circuit No. 424, or a faulty ignition module connection or module.
5. This test confirms that Code 42 is a faulty ECM and not an intermittent in circuits No. 423 or 424.

Fig 1: Code 42, Electronic Spark Timing (EST)