Code 42, Electronic Spark Timing (EST)

The ignition module sends a reference signal on circuit No. 430 to ECM when the engine is cranking. While engine speed is less than 400 RPM, the ignition module controls ignition timing. When engine speed exceeds 400 RPM, ECM applies 5 volts to the by-pass line, circuit No. 424, to switch the timing to ECM control circuit No. 423 (EST).

When engine RPM is less than 400, the ignition module grounds the EST signal and there is no voltage on the by-pass line (circuit D5). If the ECM detects voltage on the EST circuit during this condition, ECM sets a Code 42 and will not go to EST mode. With engine speed greater than 400 RPM, voltage will be applied to by-pass circuit, and EST should no longer be grounded in ignition module and EST (circuit D4) should vary.

If by-pass circuit is open or grounded, a Code 42 will set. If EST circuit is grounded, a Code 42 will set.

1. Code 42 means ECM has detected an open or short to ground in EST or by-pass circuits. This test confirms Code 42 and the fault causing the code is present.
2. Check for a normal EST ground path through ignition module. An EST circuit No. 423 shorted to ground will also read less than 500 ohms.
3. As the test light voltage touches circuit No. 424, the module should switch, causing the ohmmeter to "overrange", reading greater than 5000 ohms on the 10,000-20,000 ohm range. Ensure module switched, causing ohms to increase.
4. If the module did not switch, this test checks for:
   • EST circuit No. 423 shorted to ground.
   • By-pass circuit No. 424 open.
   • Faulty ignition module connection or module.
5. Confirms Code 42 is a faulty ECM and not an intermittent in circuits No. 423 or 424.