Code 42, Ignition Control (Ic) Circuit: Notes

The ignition module sends a reference signal on the reference high wire to ECM when the engine is cranking. When engine speed is less than about 400 RPM, the ignition module controls ignition timing. When engine speed is greater than about 400 RPM, ECM applies 5 volts to the by-pass line to switch timing control to ECM.

When engine speed is less than 400 RPM, ignition module grounds EST signal and there is no voltage on the by-pass line (circuit D5). If ECM detects voltage on EST circuit during this condition, ECM sets Code 42 and will not go to IC mode.

When engine speed is more than 400 RPM, voltage will be applied to by-pass circuit, and EST should no longer be grounded in ignition module; IC (circuit D4) voltage should vary.

If by-pass circuit is open or grounded, Code 42 will set. If IC circuit is grounded, Code 42 will set.

1. Code 42 means ECM has detected an open or short to ground in IC or by-pass circuits. This test confirms Code 42, and fault causing the code is present.
2. This step checks for a normal IC ground path through ignition module. An IC wire shorted to ground will also measure less than 500 ohms.
3. As test light voltage touches by-pass wire, module should switch, causing resistance to increase. This causes an overrange in the 1000-2000 ohm scale. If ohmmeter is in the 10,000-20,000 ohm scale, resistance of more than 5000 ohms will be indicated.
4. If the module does not switch, this step checks for the following:
   • IC circuit No. 423 is shorted to ground.
   • By-pass circuit is open.
   • Ignition module connection or module is faulty.
5. Confirms Code 42 is caused by a faulty ECM, not by an intermittent in IC or by-pass wires.