I have a computer code of 42. What does this

engine runs perfect. no misfiring at all?>??????
the computer dont know,
all it knows, is a reading fails.
in electroncs we do more testing
it can be bad wires. bad connections or bad sensors.
we use a voltmeter do check this, or a scope if fast moving signal.
1991 2.8L

"Electronic Spark Timing Circuit
Specific to 1992-93 models only.
Indicates that the ECM believes the Ignition Module has failed.
Check the wires and connectors between the Distributor and the ECM."

google finds top anwser here
https://www.2carpros.com/questions/isuzu-trooper-1992-isuzu-trooper-code-42--2

best answer ive ever seen, and for old cars, had to find,
most folks long forgot ..24 years ago
and books long ago in land fill....

Code 12 is computer is in test mode or battery has been disconnected.
Code 42 is a problem with the ASD relay circuit which could be the actual relay or something else.
Code 55 is end of test.

Code 12 is normal and indicates comms with computer are ok.
Code 34 is either:

• MAP Sensor [voltage low]
• MAF Sensor

Code 42 is for the EST circuit

You use your computer-- Google etc &

TYPE: Chevy OBD 1 Code 42

There are dozens of sites that explain
that code in lots of detail

Have a dealer or repair shop scan the truck
for body codes

They will be enhanced & not avail for free or with
a code reader

Need factory reference material to look them up &
follow diagnostic charts to resolve

CODE 11 No distributor referene signal detected during engine cranking....Check circuit between the distributer and the pcm.

CODE 12 Problem with battery connection.Direct battery input to pcm disconnected within the last 50 ignition key-on cycles.

CODE 13** Indicates a problem with map sensor vacuum system.

CODE 14** map sensor voltage to high or to low.

CODE 15** Problem with vehicle speed sensor signal.No vss signal detected during road load conditions.

CODE 17 Engine is cold to long.Engine coolant temp stays below normal operating temp(check thermostat).

CODE 21** Problem with oxygen sensor signal circuit.Sensor voltage to computer not fluctuating.

CODE 22** Coolant sensor voltage to high or to low.Test coolant temp sensor.

CODE 23** Indicates that the air temp sensor input is below the minimum acceptable voltage or sensor input is above the acceptable voltage.

CODE 24** Throttle position sensor voltage to high or low.Test the tps.

CODE 25** Idle air control (IAC) valve circuits.A shorted condition is detected in one or more of the IAC valve circuits.Or a vacuum leak has been detected.

CODE 27 One of the injector control circuit output drivers does not respond properly to the control signal.Check the circuits.

CODE 31** Problem with canister purge solenoid circuit.

CODE 32** An open or shorted condition detected in the EGR solenoid circuit.Possible air/fuel ratio imbalance not detected during diagnosis.

CODE 33 Air conditioning clutch relay circuit.An open or shorted condition in the compressor clutch relay circuit.

CODE 34 Open or shorted condition in the speed control vacuum or vent solenoid circuits.

CODE 35 Open or shorted condition detected in the radiator fan low speed relay circuit.

CODE 37** Open or shorted condition detected in the torque converter part throttle unlock solenoid control circuit.

CODE 41** Problem with charging system.An open or shorted condition detected in the generator Field control circuit.

CODE 42 Fuel pump relay or auto shutdown relay (ASD) control circuit indicates an open or shorted condition.

CODE 43** Multiple cylinder misfire detected.Peak primary circuit current not achieved with the maximum dwell time.

CODE 44 Battery temperature sensor volts malfunction.Problem with the battery temperature voltage circuit in the pcm.

CODE 46** Charging system volts to high>Computer indicates that the battery voltage is not properly regulated.

CODE 47** Charging system voltage to low.Battery voltage sense input below target charging voltage during engine operation and no significant change in voltage detected during active test or alternator output.

CODE 51** Oxygen sensor signal input indicates lean air/fuel ratio condition during operation.

CODE 52** Oxygen sensor signal input indicates rich air/fuel ratio during engine operation.

CODE 53** internal pcm failure detected.

CODE 54** No camshaft position sensor signal from distributer.Problem with the distributer synchronization circuit.

CODE 55 Completion of fault code display on check engine lamp.this is end of message code.

CODE 62 Unsuccessful attempt to update EMR mileage in the controller EEPROM.

CODE 63** Controller failure.EEPROM write denied.Check the PCM

"Code 42" would indicate that the Electronic Spark Timing (EST) signal did not change when the ECM applied bypass voltage to the ignition module. Check for faulty connections, or a faulty ignition module.

Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system. During cranking, the timing is controlled by the ignition module and the ECM grounds the EST line. It expects to see no activity on this line at this time. When the ECM enters EST mode, it applies +5 volts to the BYPASS line and expects to see voltage variations on the EST line.
Reasons for the ECM to set Code 42:
1. System in BYPASS mode (i.e. EST line supposedly grounded) but activity sensed on EST line
2. System in EST mode (i.e. BYPASS line driven with +5 volts) but no activity seen on the EST line

Possible causes include:
1. BYPASS line is open or grounded
2. EST line is open or grounded
3. Poor connections between ignition module and ECM
4. Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise
5. Faulty or incorrect ignition module
6. Faulty ECM
7. Faulty Pick Up Coil
8. Faulty Ignition Control Module
9. Faulty Crank Sensor or the Crank Sensor magnet probe could be cracked

What problems are you having currently/
12 is battery has been disconnected recently
42 is fuel pump relay circuit
45 is turbo boost, but I suspect your is non-turbo.
46 is usually charging system related.

Trouble Code 44 indicates that the O2 sensor is showing a persistently high exhaust oxygen content (lean), despite the efforts of the ECM to increase ...
Code 42 : Electronic spark timing (EST) circuit fault