HOW DO FIX A CODE #42 ON 1991 CHEVY TRUCK

Code 42 - Electronic Spark Timing Circuit (EST)

take to autoparts store, have them pull trouble code, have you replaced the OXYGEN sensor ? do it ! whens the last time you replaced both the AIR & FUEL filters ? do it ! find out code, its probably a sensor ! oxygen, like I mentioned . the MAP sensor could also be fauty . anything to do with air & fuel delivery !!! VACUUM lines, check them all, replace as necessary, TIGHTEN the mounting bolts to THROTTLEBODY, maybe the gasket is leaking ! put fuel injector cleaner in tank, next fill up . when you replaced distributor, did you replace the spark plugs & plug wires ? very important !!! EGRvalve, this valve gets clogged up severely choking the engine on these trucks !! pull it off, take a look, clean it or replace it ! when you set timing, did you follow the manual, some require you bypass the EST system, by unplugging its connection . (electronic spark timing). usually located @ firewall, under relay cover. replace the knock sensor ! todays fuel is BAD for your truck, you may think you have a miss, when you inturn have a "spark knock" , replace knock sensor @ engine block ! I think we covered it, balls in your court, hope I help you , its never just one thing, truck needs a maintenance day !!!!!!!!!

The EST if for timing ****** when a spark knock occurs it works with the knock sensor.This is not the problem what you are dealing with. if something is affecting the idle.The idle specs are 675 to 775.open the concealment plug and make your adjustment there

it is a 42, 96 on up use a OBD-II system to read codes. older than 96 use an OBD-I system to read codes. 42 means - electronic spark timing or bypass fault. this means that BYPASS line is open or grounded,
2) EST line is open or grounded
3) PROM or CALPACK not seated properly in the ECM
4) Poor connections between ignition module and ECM
5) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise)
6) Faulty or incorrect ignition module
7) Faulty ECM.

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

CODE 42 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 while the ECM monitors the engine speed. When the engine speed exceeds 400 RPM, the ECM sends a BYPASS signal to the ignition module which switches the timing to ECM control. The ECM calculates what the timing should be then "tells" the ignition module via the EST circuit.
An open or ground in the EST circuit will stall the engine and set a Code 42. The engine can be re-started but it will run on ignition module timing.
The conditions for setting this code are:

• System in BYPASS mode but the ignition module is still controlling timing
  - or -
• Engine speed > 600 RPM with no EST pulses (ECM controlled timing) going to the ignition module for 200 msec.

Typical causes for this code include:
1) BYPASS line is open or grounded
2) EST line is open or grounded
3) PROM or CALPACK not seated properly in the ECM
4) Poor connections between ignition module and ECM
5) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise)
6) Faulty or incorrect ignition module
7) Faulty ECM


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problem code 34 is in map sensor curcuit is to high. code 42 is electronic spark timing malfunction. for 42 replace est module in distributor, it has the est built into it. it controls the timing. code 34-- replacing map sensor should solve problem. good- luck!!

THE FIRING ORDER IS AS FOLLOWS : 1-6-5-4-3-2.
CYLINDER NUMBERS (front-to-rear) :
LEFT (driver side) 1-3-5
RIGHT (passenger side) 2-4-6

To set ignition timing, ensure that engine is at operating temperature, (190 degrees or above), bypass EST (electronic spark timing) by unplugging the single tan wire with a black stripe from the distributor. Ignition timing should be set at 0 degrees or TDC (top dead center). If not, loosen distributor hold down bolt and adjust, then retighten.

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

Obviously the 12 means that the system is functioning correctly Code 42 is electronic spark timing (EST) circuit grounded. Wiring issue from the harness to dist.