Federal Emissions

Code 42 Diagnostic Chart:

Ignition System Circuit Diagram:

CIRCUIT DESCRIPTION
When the system is running on the electronic ignition control module, that is, no voltage on the bypass line, the ignition control module grounds the Ignition Control (IC) signal. The 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 timing mode.

When the RPM for Ignition Control (IC) is reached (about 400 RPM), and bypass voltage applied, the IC circuit should on longer be grounded in the ignition control module so the IC voltage should be varying.

If the bypass line is open or grounded, the ignition control module will not switch to IC timing mode so the IC voltage will be low and DTC 42 will be set.

If the IC line is grounded, the ignition control module 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 ignition control module. An IC 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 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 423 shorted to ground.
^ Bypass CKT 424 open.
^ Faulty electronic ignition control module connection or module.

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


DIAGNOSTIC AIDS:

^ The Tech 1 (or equivalent) scan tool does not have any ability to help diagnose a DTC 42 problem.

^ A PROM not fully seated in the ECM can result in a DTC 42.

^ Refer to "Intermittents" in "Symptoms".