DTC 42

Fig. 023 Chart - Code 42 Electronic Spark Timing:

Fig. 024 - Wiring Diagram for Code 42 Electronic Spark Timing. VIN 1:

CIRCUIT DESCRIPTION:
The DIS module sends a reference signal to the ECM when the engine is cranking. While the engine speed is under 400 rpm, the DIS module controls the ignition timing. When the system is running on the ignition module (no voltage on the Bypass line), the ignition module grounds the EST signal. The ECM expects to sense no voltage on the EST line during this condition. If it senses a voltage, it sets Code 42 and will not enter the EST mode.
When the engine speed exceeds 400 rpm, the ECM applies 5 volts to the Bypass line to switch the timing to ECM control (EST). If the Bypass line is open or grounded, once the rpm for EST control is reached, the ignition module will not switch to EST mode. This results in low EST voltage and the setting of Code 42. 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 sensed 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 500 ohms, but this will be checked later.
3. As the test light voltage contacts 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 a reading above 5000 ohms. The important thing is that the module "switched."
4. The module did not switch and this step checks for
a. EST CKT 423 shorted to ground
b. Bypass CKT 424 open.
c. Faulty ignition module connection or module
5. Confirms that Code 42 is a faulty ECM and not an intermittent in CKTs 423 or 424

DIAGNOSTIC AIDS:
The "Scan" tool does not have any ability to help diagnose a Code 42 problem. If Code 42 is intermittent, refer to DIAGNOSIS BY SYMPTOMS.