Code EO42, Est Circuit Open Or Grounded: Notes

Code EO42 will set if EST or by-pass circuit is open or grounded at time of engine start-up.

1. Tests if ECM recognizes a problem. If it doesn't set Code EO42 at this point, it is an intermittent problem. Check for a loose connection.
2. With the ECM disconnected, the digital volt-ohmmeter should read less than 200 ohms. This is normal EST circuit resistance through the ignition module. A higher resistance would indicate a fault in circuit No. 423, a poor ignition module connection or a faulty ignition module.
3. If test light was on when connected from 12 volts to ECM harness by-pass circuit, either circuit No. 424 is shorted to ground or the ignition module is faulty.
4. Tests if ignition module switches when by-pass circuit is energized by 12 volts through test light. If ignition module switches, the resistance reading should switch from less than 200 ohms to more than 6000 ohms.
5. Disconnecting the ignition module should make the ohmmeter read as if it were monitoring an open circuit (infinite reading). Otherwise, circuit No. 423 is shorted to ground.