DTC 42

Ignition System Circuit Diagram:

DESCRIPTION

When the system is running on the ignition module, that is, no voltage signal on the bypass line (module mode), the module grounds the EST circuit. The PCM expects to see zero volts on the EST circuit during this condition. If it sees a voltage, Code 42 is set and the PCM will not switch to EST mode.

When the specified engine speed for EST mode is reached (about 400 rpm), and bypass voltage is applied to the bypass circuit, the EST should no longer be grounded and ECT voltage signal should be varying. If the EST line is grounded, the ignition module will still switch to EST mode, but because the line is grounded, there will be no EST signal, and Code 42 will set.

If the bypass line is open or grounded, the ignition module will not switch to EST mode, the EST voltage will remain low and Code 42 will set.

TEST DESCRIPTION

Code 42 Diagnostic Chart:

Numbers below refer to circled numbers on the diagnostic chart.
1. Code 42 means the PCM 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 normal EST circuit ground path through the PCM. An EST circuit 423 shorted to ground will also read less than 500 ohms. This condition will be tested for later in the procedure.
3. As the power from the test light is applied to circuit 424, the module should switch, causing the ohmmeter to "over-range" (if the meter is in the 1K - 2K ohm range). Selecting the 10K - 20K range should read a resistance of over 5K ohms. The important thing is that the module switched.
4. The module did not switch and this step checks for:
^ EST circuit 423 shorted to ground.
^ Bypass circuit 424 open.
^ Faulty ignition module connection or faulty module.

5. Confirms that Code 42 is due to a faulty PCM and not intermittent problem in circuits 423 or 424.

DIAGNOSTIC AIDS
^ A "SCAN" tool will not help in diagnosing Code 42 problems.
^ An incorrectly installed MEM-CAL (not fully seated in the PCM) can cause a Code 42.