G. M.’s Misunderstood Codes CODE 42


Staff member
Nov 13, 2006
By Roy, BAT Auto Technical,Technical Editor

Probably the most misunderstood code (s) are the ones that deal with quad driver circuit faults like 26, 27, 28, and 29. These are covered in part 2 of my G.M. ECM RESETS article.

Another code that seems to bring about quite a bit of confusion is code 42 found on most G.M. vehicles except some Cadillac’s and other E-body vehicles that use code (E) 23 as a flag for an Electronic spark timing (EST) circuit fault.

The EST circuit has been on G.M. vehicles for quite a few years. So, EST can be seen on a vehicle with a high-energy ignition distributor (HEI) or, vehicles that are equipped with a direct ignition system. (DIS) {Distributor –less}
Why EST?
To provide improved engine performance, better fuel economy, and to better control exhaust emissions, the computer controls spark advance (timing) with the EST system. To calculate spark advance the computer uses information from the manifold absolute pressure (MAP) sensor and the coolant temperature sensor (CTS) as well as engine RPM.
How spark advance is calculated:
· 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 system.
The EST system consists of the ignition module, computer (ECM/PCM), and the connecting wiring. There are four circuits to this system.
The circuits and their functions:
· 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/E23.

· 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.
Why the computer flags this code.
What we have here is a failure to communicate. The ignition module and the computer are in constant communication when the engine is in operation. (Over 400 RPM.)With the computer telling the module the correct timing. If communication is lost or a circuit becomes infected with false signals (bad grounds, short to battery +, Electro Magnetic Interference {spark plug wires to close to circuit wiring}, oil contamination of the circuit connectors and wiring) then the computer will turn timing control back to the ignition module and flag the code.
Possible causes:
· 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.

If you are testing a vehicle for this code check the routing of the spark plug wires making sure they are not to close to the EST circuit(s) wiring. Check the connector terminals making sure they are not over-sized or oil contaminated. Check the entire wiring of the circuits for chaffing,
short(s) ground. Alldata has a very good on-line manual system for the DIY’s that contains all the information and a trouble “tree” chart(s) for the proper method to test the ignition module and computer for your vehicle.

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