Without 4L60E/4L80E Automatic Transmission

Electronic Spark Timing Circuit Code 42:

Ignition Control (IC) Without 4L80-E Transmission (ECM):

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 10,000 to 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.

5. Confirms that Code 42 is a faulty ECM and not an intermittent in CKTs 423 or 424.

Diagnostic Aids:
The Tech 1 does not have any ability to help diagnose a Code 42 problem. - Intermittent Malfunctions