code42

[BEAVIS18]

I have a 92 with 3.1 i have a code 42 and don't know how to go about testing it so if anyone could help me with it that would be good

[Blue Bowtie]

Code 42 Error Diagnosis

The computer uses information from the Throttle Position Sensor (TPS), Manifold Absolute Pressure (MAP) sensor, Coolant Temperature Sensor (CTS), and engine RPM as monitored by the reference pulses to calculate correct spark advance.

Ignition advance is affected by several factors:

Low MAP (low engine load) = more spark advance
Cold engine = more spark advance
High MAP (high engine load) = less spark advance
Warm engine = less spark advance
High RPM = more spark advance
Low RPM = less spark advance

The complete EST system consists of the ignition module, computer (ECM/PCM), and the connecting wiring. There are four circuits to this system:

Reference:
This circuit provides the computer with RPM information. If this circuit becomes open or shorted to ground the engine will not run, because the computer will not operate “trigger” the fuel injectors. A common saying here is, no reference no run.

Reference ground:
This circuit is grounded in the ignition module and the computer; it makes sure the ground circuit has no voltage drop, which could affect vehicle performance. I have found an open in this circuit can cause a vehicle “no start” as well as a false code 42.

By-pass:
The computer applies 5 volts to this circuit to switch timing control from the ignition module to the computer. The computer is telling the ignition module that it will control the timing. An open or a short to ground of this circuit will set the code and the engine will run on base timing. (Plus a small amount of advance built into the ignition module.)

EST:
This circuit triggers the ignition module. The computer does not know what the actual timing is, what it does know is when it gets a reference signal. It uses this reference signal as a base point and advances or retards the spark from that point. So, if on vehicles where the base timing is adjustable, and the base timing is set incorrectly, the spark curve will be off through its entire range by the amount that the base timing is misadjusted. An open or short to ground in this circuit will set the code and cause the engine to run on ignition module timing.


The ignition module and the computer are in constant communication when the engine is in running at anything over 400 RPM. The ECM provides a signal to the ESC for the correct timing advance. If communication is lost or a circuit becomes infected with false signals from bad grounds, short to power sources, electromagnetic interference, RFI, or oil contamination of connectors or wiring, the computer will turn timing control back to the ignition module and set the error code. The common possible causes for this are:

EST circuit open or grounded.
Bypass circuit open or grounded.
Reference Ground circuit open or shorted to battery +.
A bad computer to engine circuit ground.
A defective ignition module.
A defective computer.

Check spark plug wire routing to making sure they are not to close to EST circuit wiring. Check connector terminals for distortion, looseness, or contamination. Check the entire wiring of the circuits for shorts and grounding.