Code 42: Electronic Spark Timing: Notes

When the system is running on the ignition module (cranking/no voltage on the by-pass line), ignition module grounds the Electronic Spark Timing (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 EST is reached (about 400 RPM), and by-pass voltage is applied, 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 then 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 fault causing the code is still present.
2. Checks for a normal EST ground path through the 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, module should switch causing the ohmmeter to "overrange" if the meter is in the 1000-2000 ohms position. Selecting the 10,000-20,000 ohms position will indicate greater than 5000 ohms. The important thing is that the module is "switched".
4. The module did not switch and this test checks for EST circuit No. 423 shorted to ground, by-pass circuit No. 424 open and a faulty ignition module connection or module.
5. Confirms that Code 42 is a faulty ECM unit and not an intermittent in circuits No. 423 or 424.