Code 42
Code 42 is a diagnostic trouble code related to the Electronic Spark Timing circuit on the Pontiac Fiero.
Code 42 indicates that the ECM has detected a problem in the Electronic Spark Timing system, commonly involving the EST circuit, bypass circuit, ignition module, ECM, wiring, or connectors.
Overview
The Fiero ignition system uses Electronic Spark Timing, often abbreviated as EST.
Under normal operation, the ECM can control ignition timing through the ignition module and related wiring. Depending on engine and ignition system, this may involve a distributor-mounted ignition module or a distributorless ignition module.
Code 42 is set when the ECM detects an unexpected condition in the Electronic Spark Timing circuit. This may involve the ignition module, ECM, bypass circuit, EST circuit, wiring, connectors, or related ignition-control circuits.
Code 42 should not automatically be treated as a failed ignition module. The ignition module is a common suspect, but wiring and connector faults can produce the same code.
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Flash pattern
Code 42 is displayed as:
- Four flashes
- Pause
- Two flashes
This represents code 42.
Animated flash pattern:
Meaning
Code 42 usually indicates an Electronic Spark Timing circuit problem.
Possible causes include:
- Faulty ignition module
- Poor ignition module connector contact
- Open or grounded bypass circuit
- Open or grounded EST circuit
- Ignition module wiring problem
- Distributor connector problem, where applicable
- ECM wiring problem
- Poor ECM connector contact
- Faulty ECM
- Incorrect or damaged ignition wiring
- Poor grounds
- Previous-owner wiring modifications
- PROM or ECM-related connection issue, where applicable
Because the EST system depends on communication between the ignition module and ECM, a failure in either the module side or ECM-side wiring can set the code.
Electronic Spark Timing operation
Electronic Spark Timing allows the ECM to control ignition timing during normal engine operation.
On distributor ignition systems, the ignition module initially controls spark timing during cranking and very low engine speed. After engine speed rises above the bypass threshold, the ECM switches to EST control and commands ignition timing through the EST circuit.
The system commonly involves:
- Reference signal
- Reference ground
- Bypass circuit
- EST circuit
- Ignition module
- ECM
- Related wiring and connectors
The reference signal provides the ECM with engine speed and crankshaft-position information. The bypass circuit is used to switch timing control from the ignition module to the ECM. The EST circuit is used by the ECM to control the ignition module.
If the bypass or EST circuit is open, grounded, shorted, or not switching correctly, Code 42 may be set.
Bypass circuit
The bypass circuit is used to switch ignition timing control from module-controlled timing to ECM-controlled timing.
At low engine speed, the ignition module controls timing. Once the engine reaches the bypass threshold, the ECM applies voltage to the bypass circuit so timing control can transfer to the ECM.
If the bypass circuit is open or grounded, the ignition module may not switch properly to EST mode. This can set Code 42 and may leave the engine running on base module timing rather than normal ECM-controlled timing.
Symptoms may include poor performance, poor fuel economy, hesitation, or timing-related drivability problems.
EST circuit
The EST circuit is used by the ECM to trigger or control the ignition module during ECM-controlled timing operation.
If the EST circuit is open, shorted, grounded, or otherwise not behaving as expected, the ECM may set Code 42.
Because the ECM does not directly know actual spark timing, correct base timing and proper EST circuit operation are important.
Distributor ignition systems
On V6 Fieros with distributor ignition, the EST system works with:
- Distributor-mounted ignition module
- Distributor pickup coil
- Ignition coil
- Distributor connector
- ECM
- Reference circuit
- Bypass circuit
- EST circuit
- Related wiring and grounds
On these systems, Code 42 is often discussed together with Ignition module failure, but the ignition module is only one possible cause.
A bad pickup coil, poor distributor connector, damaged wiring, poor ECM connection, or poor ground can cause similar symptoms or contribute to the diagnosis.
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Distributorless ignition systems
Some four-cylinder Fieros use a distributorless ignition system rather than a conventional distributor.
On distributorless systems, the ignition module, crankshaft sensor, coil packs, ECM, and EST-related circuits perform the ignition-control functions.
Because the hardware is different, Code 42 diagnosis on a distributorless system should use the correct diagnostic information for that ignition system.
Do not assume V6 distributor-module diagnosis applies directly to a distributorless four-cylinder system.
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Symptoms
Possible symptoms may include:
- Check engine light
- Stored Code 42
- No-start condition
- Hard starting
- Stalling
- Intermittent shutdown
- Engine starts but runs poorly
- Poor drivability
- Misfire
- Poor acceleration
- Poor fuel economy
- Timing-related drivability problems
- Engine appears to run on base timing
- Hot stall or hot restart problem, if related to ignition module or wiring
Some cars may set Code 42 with only minor drivability symptoms, while others may run poorly or fail to start depending on the fault.
Diagnosis
Basic diagnostic checks include:
- Confirm that Code 42 is present.
- Check whether other diagnostic trouble codes are also present.
- Determine which ignition system the car has.
- Inspect the ignition module connectors.
- Inspect distributor or DIS module wiring, depending on system.
- Check for damaged, loose, corroded, or overheated terminals.
- Check ignition power and ground.
- Check ECM grounds and ECM connector condition.
- Inspect the bypass and EST circuits.
- Check for previous wiring repairs or modifications.
- Check ignition module operation.
- Check pickup coil or crankshaft sensor operation, depending on system.
- Verify ignition timing and related ignition-system condition if applicable.
- Follow the correct diagnostic chart for the engine and ignition system.
Do not diagnose Code 42 by replacing the ignition module first unless testing or substitution is being used as part of a controlled diagnosis.
Ignition module relationship
The ignition module is a common part to inspect when diagnosing Code 42.
However, Code 42 should not automatically be treated as a failed ignition module.
Wiring, connectors, grounds, ECM connections, bypass circuit faults, EST circuit faults, pickup coil problems, crank sensor problems, and related ignition components should also be checked.
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Pickup coil or crank sensor relationship
The ignition module needs an engine-position signal to control spark and provide reference information.
On distributor ignition systems, this signal comes from the pickup coil in the distributor.
On distributorless ignition systems, the crankshaft sensor is part of the ignition-module input system.
A weak, intermittent, or failed pickup coil or crankshaft sensor can create symptoms similar to ignition module or EST problems.
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ECM relationship
The ECM is part of the EST system.
A Code 42 can involve ECM-side wiring, ECM connector problems, ground problems, PROM/CALPAK seating issues where applicable, or the ECM itself.
The ECM should not be blamed until ignition module connections, bypass and EST wiring, grounds, power feeds, and related ignition components have been checked.
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No-start relationship
Code 42 can appear together with no-start or stall complaints, but it is not the only possible no-start cause.
A crank/no-start diagnosis should also check:
- Spark
- Injector pulse
- Fuel pressure
- ECM power
- Ignition power feed
- Reference signal
- Pickup coil or crank sensor signal
- Ignition coil
- Module connectors
- ECM connectors
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Intermittent faults
Code 42 can be intermittent.
Intermittent Code 42 problems may be caused by:
- Heat-sensitive ignition module
- Loose ignition module connector
- Broken pickup-coil wires
- Heat-damaged wiring
- Poor ECM connector contact
- Poor ground
- Vibration-sensitive wiring fault
- Previous wiring repair
- Moisture or corrosion in connectors
If the car stalls when hot and restarts after cooling, the ignition module, pickup coil, wiring, and related connectors should be inspected carefully.
Common mistakes
Common mistakes when diagnosing Code 42 include:
- Replacing the ECM before checking the ignition module and wiring
- Replacing the ignition module without checking connectors
- Confusing Code 42 with a simple timing-adjustment issue
- Ignoring pickup coil or crank sensor problems
- Ignoring poor grounds
- Ignoring previous-owner wiring modifications
- Assuming the same test procedure applies to distributor and distributorless ignition systems
- Setting base timing incorrectly and then blaming the ECM
Service notes
When diagnosing Code 42:
- Use the correct diagnostic information for the engine and ignition system.
- Identify whether the car has distributor ignition or distributorless ignition.
- Inspect connectors before replacing parts.
- Check the EST and bypass circuits carefully.
- Check ignition power and ground.
- Check ECM grounds and connectors.
- Use proper heat-sink compound if replacing a distributor-mounted ignition module.
- Do not substitute ordinary dielectric grease for ignition-module heat-sink compound.
- Verify base timing if timing-related work has been performed.
- Do not assume every Code 42 is caused by the ignition module.
Notes
Code 42 can be caused by the ignition module, but it can also be caused by wiring, connectors, grounds, ECM-side faults, or related ignition-system components.
A Code 42 diagnosis should focus on the Electronic Spark Timing system as a circuit, not just on one replaceable part.