DTC 42

Code 42 Wiring Diagram:

Code 42 - Electronic Spark Timing (EST) Circuit:

CIRCUIT DESCRIPTION
When the system is running on the ignition module, that is, no voltage on the bypass line, the ignition module grounds the EST signal. The ECM expects to see no voltage on the EST line during this condition. If it 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 bypass voltage applied, the EST should no longer be grounded in the ignition module so the EST voltage should be varying.
If the bypass line is open or grounded (as when timing is set), 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.

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 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 D4, 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:
- EST CKT D4 shorted to ground
- Bypass CKT D5 open
- Faulty ignition module connection or module.
5. Confirms that Code 42 is a faulty ECM and not an intermittent in CKT D4 or CKT D5.

DIAGNOSTIC AIDS:
If a Code 42 was stored and the customer complains of a "Hard Start," the problem is most likely a grounded EST line (CKT D4).
A PROM not fully seated in the ECM can result in a Code 42. A Code 42 will set if base timing was checked or set by grounding the set timing connector.
See DIAGNOSIS BY SYMPTOM - NO TROUBLE CODES STORED.