EST Circuit

CODE 42: EST IGNITION CIRCUIT
To set a Code 42 one of the following conditions must occur:
Engine speed greater than 600 RPM with no EST pulse for 100-200 milliseconds (open or grounded circuit No. 423).
ECM commanding by-pass mode (open or grounded circuit No. 424).
NOTE:Test numbers refer to test numbers on diagnostic chart.

Tests if ECM recognizes a problem. If it doesn't set Code 42 at this point, it is an intermittent problem. Check for a loose connection.
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.
If test light was on when connected from 12 volts to ECM harness terminal BC7, either circuit No. 424 is shorted to ground or the ignition module is faulty.
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 shift to more than 6000 ohms.
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.
Diagnostic Aids
An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation.
Poor Connection - Inspect ECM harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection or damaged harness.
Intermittents - If connections and harness are okay, connect a digital volt-ohmmeter between affected terminal to ground and monitor meter while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change.