The ignition module is the most common failure for this code but needs to be confirmed.
I attached a flow chart and wiring diagram for you to view to determine the failure.
When the system is running on the Ignition Module, that is, no voltage on the by-pass line, the Ignition Module grounds the EST signal. The ECM expects to see no voltage on the EST line during this condition. If is sees a voltage, it sets Code 42 and will not go into the EST mode.
When the rpm for EST is reached (about 400 rpm), and by-pass voltage applied, the EST should no longer be grounded in the ignition module so the EST voltage should be varying.
If the by-pass line is open or grounded, the Ignition Module will not switch to EST mode so the EST voltage will be low and Code 42 will be set.
If the EST line is grounded, the Ignition Module will switch to EST, but because the line is grounded there will be no EST signal. A Code 42 will be set.
Code 42 sets if there is an open or a short to ground in the EST or by-pass circuit.
Test Description: Numbers below refer to circled numbers 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 10-20,000 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.
The "Scan" tool does not have any ability to help diagnose a Code 42 problem. See Diagnosis By Symptoms - No Trouble Codes Stored.
Images (Click to enlarge)
Tuesday, May 14th, 2019 AT 7:00 AM