DTC 42

Code 42 - C3-I EST or Bypass Circuit Failure:

Wiring Diagram for CAM Sensor Circuit:

CODE 42 - C3-I EST OR BYPASS CIRCUIT FAILURE

Circuit Description:

The Computer Controlled Coil Ignition C3-I module sends a reference signal to the Electronic Control Module (ECM), when the engine is cranking. While the engine is under 400 rpm, the C3-I module controls the ignition timing. When the engine speed exceeds 400 rpm, the ECM sends a 5 volts signal on the bypass CKT 424 to switch the timing to ECM control, through the Electronic Spark Timing (EST) CKT 423. An open or ground in the EST circuit will stall the engine and set a Code 42. The engine can be restarted but will run on module timing.

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

^ Engine speed greater than 600 rpm with no EST pulse for 200 ms. (Open or grounded CKT 423) or,

^ ECM commanding bypass mode. (Open or grounded CKT 424.)


Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. Checks to see if ECM recognizes a problem. If it doesn't set Code 42 at this point, it is an intermittent problem and could be due to a loose connection.

2. With the ECM disconnected, the ohmmeter should be reading less than 200 ohms which is the normal resistance of the EST circuit through the C3-I module. A higher resistance would indicate a fault in CKT 423 a poor C3-I module connection, or a faulty C3-I module.

3. If test light was "ON", when connected from 12 volts to ECM harness terminal "D5" either CKT 423 is shorted to ground or the C3-I module is faulty.

4. Checks to see if C3-I module switches when the bypass circuit is energized by 12 volts through the test light. If the C3-I module actually switches, the ohmmeter reading should shift to over 8,000 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 ECM harness connectors for backed out terminals, at "B4" or "D5", improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.

^ Intermittent Test - If harness and harness connections check OK, using a DVM connected to affected terminal and ground, note DVM while moving related connections and wiring harness. If the failure is induced, the voltage reading will change.