DTC 42

Engine Cranks But Won't Run Circuit:

Circuit Description:
The ignition module sends a spark reference and a fuel control reference signal to the PCM when the engine is cranking. At the start of crank, the ignition module controls ignition timing (module mode). When the PCM recognizes the second fuel control pulse., it sends a 5 volts signal on the bypass CKT 424 to switch timing to PCM control. An open or ground in the EST or bypass circuit at the time the engine is started will set a Code 42. The engine may be started but will run on module mode timing.

To set a Code 42 the following conditions must be met:

^ Open or grounded CKT 423 at the time of engine start-up.

OR

^ PCM commanding bypass mode (open or grounded CKT 424) when the engine is started.

Code 42 Diagnostic Chart:

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Checks to see if PCM recognizes a problem. If it does not set Code 42, it is an intermittent problem and could be due to a loose connection.

2. With the PCM disconnected, the ohmmeter should be reading less than 200 ohms, which is the normal resistance of the EST circuit through the ignition module. A higher resistance would indicate a fault in CKT 423, a poor ignition module connection, or a faulty ignition module.

3. If test light was "ON" when connected from 12 volts to PCM harness terminal "BC 6". either CKT 424 is shorted to ground or the ignition module is faulty.

4. Checks to see if ignition module switches when the bypass circuit is energized by 12 volts through the test light. If the ignition module switches, the ohmmeter reading should shift to over 6.000 ohms.

5. Disconnecting the ignition module should make the ohmmeter read as if it were monitoring an open circuit (infinite reading). Otherwise, CKT 423 is shorted to ground.


Diagnostic Aids:
An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation.

Check For:

^ Poor Connection or Damaged Harness - Inspect PCM harness connectors for backed out terminals "BC6" or "BC7", improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

^ Intermittent Test - Monitor a digital voltmeter connected from affected terminal to ground while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change.

^ Detonation/Spark Knock/Pings