Code 42: Est Ignition Circuit: Notes

CAUTION: Some vehicles may have different wire colors. See TSB No. 89-6E-9

To set a Code 42 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).
OR: ECM commanding bypass mode (open or grounded circuit No. 424).

NOTE: Test numbers refer to test numbers on diagnostic chart.

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.
With the ECM disconnected, the ohmmeter should be reading less than 200 ohms, which is the normal resistance of the EST circuit 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. 423 is shorted to ground, or the ignition module is faulty.
Checks to see if ignition module switches when the bypass circuit is energized by 12 volts through the test light. If the ignition module actually switches, the ohmmeter reading should shift to greater 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.