Code 42 - Est (Electronic Spark Timing): Notes

DIS module sends a reference signal to ECM when engine is cranking. When engine speed exceeds 500 RPM, ECM applies 5 volts to by-pass line to switch timing to ECM control. When system is running on DIS module (no voltage on by-pass line), DIS module grounds the Electronic Spark Timing (EST) signal. The ECM expects to see no voltage on EST line during this condition. If ECM sees a voltage signal, it sets Code 42 and will not go into EST mode. When RPM for EST is reached (approximately 500 RPM) and by-pass voltage is applied, EST should no longer be grounded in DIS module, so EST voltage will vary. If by-pass line is open or grounded, DIS module will not switch to EST, so EST voltage will be low and Code 42 will set. If EST line is grounded, DIS module will switch to EST, but because line is grounded, there will not be an EST signal, and Code 42 will set.

NOTE: Test numbers refer to numbers on diagnostic chart.

Code 42 indicates ECM has seen an open or short to ground in EST or by-pass circuit. This confirms Code 42 and fault causing code are still present.
This checks for normal EST ground path through ignition module. An EST circuit shorted to ground will also read less than 500 ohms.
As test light voltage touches by-pass circuit, module should switch, causing ohmmeter to indicate an over-range (if ohmmeter is adjusted to the 1000-2000 ohms range). Selecting the 10,000-20,000 ohms range will indicate greater than 5000 ohms. The important thing is that module is switched.
Module does not switch. This checks for EST circuit shorted to ground, by-pass circuit open or a faulty ignition module connection or module.
This confirms Code 42 is caused by faulty ECM, not an intermittent circuit.