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.
Diesel Diagnostic Check - Tester Check
This check determines if the ECM is capable of detecting a fault in the DEC system. Prior to any diagnostics, correct operation of the tester must be verified.
- This step checks to see if the tester is supplied with 12 volts. With power cord installed, light should glow.
- This step determines if the ALDL circuit is grounded or faulty. When the tester is connected to the ALDL connector, the "CHECK ENGINE" light (CEL) should remain on.
- When ignition is turned on, the CEL should remain on. If not, ECM may be shorted internally. The normal ECM reset time is 10 seconds. Therefore, when the tester is used during diagnostics, the CEL will flicker when function knob is rotated.
Use the Manual With the Right Hardware
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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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