Code 42 - Electronic Spark Timing (Est): Notes

When the system is running on the ignition module, that is, no voltage on the by-pass line, the ignition module grounds the EST signal. The ECM expects to see no voltage on the EST line during this condition. If the ECM sees a voltage signal, it sets Code 42 and will not go into the EST mode.

When the rpm for ESTR is reached (about 400 rpm), and by-pass voltage is applied, the EST should no longer be grounded in the ignition module so the EST voltage should be varying.

If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded there will be no EST signal. A Code 42 will be set.

1. Code 42 means the ECM has seen an open or short to ground in the EST or by-pass circuits. This test confirms Code 42 and that the fault causing the code is present.
2. Checks for a normal EST ground part through the ignition module. An EST circuit No. 423 shorted to ground will also read less than 500 ohms, however, this will be checked later.
3. As the test light voltage touches circuit No. 424, the module should switch causing the ohmmeter to "overrange". If the meter is in the 1000-2000 ohms position, selecting the 10-20,000 ohms position will indicate above 5000 ohms. The important thing is that the module is "switched".
4. The module did not switch and this test checks for the following conditions:
   • EST circuit No. 423 shorted to ground.
   • By-pass circuit No. 424 open.
   • Faulty ignition module connection or module.
5. Confirms that Code 42 is a faulty ECM unit and not as intermittent in circuits No. 423 or 424.