dbrunner2

Well, I finally got a copy of the diagnostic chart for Code 42 on the LT1, and I thought it might be helpful to provide an electronic copy so people won't need to spend so much time hunting for the chart. The copy I have appears to have come from a manual for 92-93 LT1s, but from what I understand it should apply to all LT1s. I've clarified and annotated the original text in some places.

Test A (start here)
Clear DTC(s) and crank engine for 15 seconds. Does DTC 42 set? If yes, go to test B; if no, DTC 42 is intermittent.
Note: DTC 42 will set if voltage on white wire from ignition module to ECM/PCM is below about 0.5 volt while engine speed is below 3000 rpm, or the high and low resolution signals from the Optispark are shorted together. When either of these conditions is present, the ECM/PCM disables the injectors to avoid flooding.

Test B
With ignition off, disconnect ignition coil module connector, and connect a voltmeter, set to AC scale, between ICM harness connector terminal B (white wire) and ground. Crank engine and observe voltage. Is voltage between 1 and 4 volts? If yes, go to test C; if no, go to test D.
Note: This check determines if the IC signal from the ECM/PCM is available at the ignition coil module.

Test C
With ignition off, connect test light between battery positive terminal and ICM harness connector terminal C (black wire). Is light on? If yes, go to test E; if no, ICM ground circuit is open (black wire is broken or not properly connected to ground).
Note: The remaining tests begin to check that the ignition coil module circuitry is OK. If the ignition coil module loses its voltage source, secondary voltage will not be produced and a DTC 42 will set.

Test D
With ignition off, disconnect ECM/PCM connector A. Connect test light between battery positive terminal and ECM/PCM harness terminal A12 (white wire to ICM). Is test light on? If yes, the white wire is grounded somewhere between the ECM/PCM and the ICM; if no, either the white wire is broken somewhere between the ECM/PCM and the ICM or the ECM/PCM is defective.

Test E
With ignition on, connect a test light between ICM harness connector terminal A (pink/black wire) and ground, and then between terminal D (white/black) and ground. Is the light on in both cases? If yes, the ICM or its connector is faulty; if no, the connection from the ICM to the coil is broken on the circuit/circuits that did not light.
Note: since the ignition coil module gets its power from the coil, check the ignition feed circuit to the ignition coil for opens.

uNkleStv

iTrader: (1)

Nice write up. I just picked up a 94 z, and it's tossing the code 42. Hopefully the procedure will lead me to the problem.

Thanks for sharing!

uNkleStv

iTrader: (1)

I had a bad ICM, the above write up nailed it down in 10 minutes.

Thanks OP!!!!!!

BlackWidowZ28

So in my last test I get no power to either wire. What is my next step?

jimdee

all tests were good icm tested good replaced connector still get code 42.