DTC 42

DIAGNOSTIC CHART





WIRING DIAGRAM

INSPECTION PROCEDURE

Circuit Description:
When the system is running on the Ignition Control Module (ICM), that is, no voltage on the bypass line, the ICM grounds the IC signal. The Engine Control Module (ECM) expects to see no voltage on the IC line during this condition. If it sees a voltage, it sets DTC 42 and will not go into the IC mode.
When the rpm for IC is reached (about 400 rpm), and bypass voltage applied, the IC should no longer be grounded in the ignition module so the IC voltage should be varying.
If the bypass line is open or grounded, the ignition module will not switch to IC mode so the IC voltage will be low and DTC 42 will be set.
If the IC line is grounded, the ICM will switch to IC, but because the line is grounded, there will be no IC signal. A DTC 42 will be set.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.
1. DTC 42 means the ECM has seen an open or short to ground in the IC or bypass circuits. This test confirms DTC 42 and that the fault causing the DTC is present.
2. Checks for a normal IC ground path through the 1CM. An IC CKT D4 shorted to ground will also read less than 500 ohms; however, this will be checked later.
3. As the test light voltage touches CKT D5, the module should switch causing the ohmmeter to overrange' if the meter is in the 1000-2000 ohms 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:
- IC CKT D4 shorted to ground.
- Bypass CKT D5 open.
- Faulty CM connection or module.
5. Confirms that DTC 42 is a faulty ECM and not an intermittent in CKTs D4 or D5.

Diagnostic Aids:
If a DTC 42 was stored and the customer complains of a "hard start", the problem is most likely a grounded IC line (CKT D4).
The "Scan" tool does not have any ability to help diagnose a DTC 42 problem.
A Programmable Read Only Memory not fully seated in the ECM can result in a DTC 42.
Refer to "Intermittents" in "Symptoms".