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.
Code 42, Est Circuit Open Or Grounded: Notes
Code 42 indicates the ECM has seen an open or short to ground in HEI EST or by-pass circuit.
- 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 the ignition module. If EST circuit is shorted to ground, reading will be less than 500 ohms.
- As test light voltage touches by-pass circuit, the module should switch. This will cause the ohmmeter to switch from hundreds of ohms to thousands of ohms. This test assures that the module "switched".
- If module did not switch, this step will test for a short in circuit No. 423, an open in circuit No. 424, and a faulty ignition module connection or module.
- This step confirms that Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.
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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.
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