Code 42 - Est Circuit Open Or Grounded: Notes

The ignition module sends a reference signal to ECM when engine is cranking. While engine is less than 700 RPM, ignition module controls ignition timing. After engine speed exceeds 700 RPM, ECM sends a 5 volt signal on the by-pass circuit No. 424 to switch timing to ECM control through EST circuit No. 423. An open or ground in the EST or by-pass circuit will set a Code 42 and cause engine to run on module or by-pass timing. This will result in poor performance and poor fuel economy.

1. This test checks to see if ECM recognizes a problem. If a Code 42 has not been set at this point, problem is intermittent and could be result of a loose connection.
2. With ECM unplugged, ohmmeter should read less than 500ohms, which is normal resistance of ignition module. Higher resistance would indicate a fault in circuit No. 423, poor ignition module connection or faulty ignition module.
3. If test light illuminates when connected from 12 volts to ECM harness terminal "BC7", either circuit No. 423 is shorted to ground or ignition module is faulty.
4. Check to see if ignition module switches when by-pass circuit is energized by 12 volts through test light. If module actually switches, ohmmeter reading should shift to greater than 8000 ohms.
5. Unplugging ignition module should cause ohmmeter to read as if it were monitoring an open circuit (infinite reading). If ohmmeter has a reading other than infinite, circuit No. 423 is shorted to ground.