Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
SYS.OK|MANUAL.DB● LIVE
HOMESERVICE MANUALSPONTIAC1991FIREBIRD FORMULA, 5.0 F, STANDARD, M39REPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 15 (ENGINE CONTROLS - TESTS W/CODES - 3.1L PFI)PCM/BCM CODE CHARTSCODE 42, EST CKT OPEN OR GROUNDEDNOTES
1991 Pontiac Firebird Formula, 5.0 F, Standard, M39
Code 42, Est CKT Open Or Grounded: Notes
1991 Pontiac Firebird Formula, 5.0 F, Standard, M39SECTION Notes
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 1992 Pontiac Sunbird, 1992 Pontiac Grand Prix, 1992 Pontiac Firebird, 1992 Oldsmobile Cutlass Supreme, and 1992 Chevrolet Lumina. However, it is still accessible from the selected car via links, so may be relevant.
Code 42 indicates ECM has seen an open or short to ground in HEI EST or by-pass circuit.
NOTE:
Test numbers refer to numbers on diagnostic chart.
- This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
- This test checks for a normal EST ground path through ignition module. If EST circuit is shorted to ground, reading will be less than 500 ohms.
- As test light voltage touches by-pass circuit, module should switch. This will cause ohmmeter to switch from hundreds of ohms to thousands of ohms. This test assures module "switched".
- If module did not switch, this tests for a short in EST circuit, an open in by-pass circuit and a faulty ignition module connection or module.
- This step confirms Code 42 is a faulty ECM and not an intermittent problem in EST or by-pass circuits.
RENDER: 1.0x
NO RELATED
Recommended Tools & Savings
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.