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.
Diagnostic Circuit Check
Diagnostic circuit check is the starting point for identifying a problem caused by the engine control system. ECM uses the CHECK ENGINE light to flash fault code(s). The lowest code number will be flashed first. If no codes were stored, see TESTS W/O CODES article in this section.
- Checks the CHECK ENGINE light operation.
- Checks to see if ECM self-diagnostic mode is operating.
- Checks to see if ECM serial data output is operating.
- Checks to see if vehicle will start.
- Checks for any codes that are stored in the ECM memory with engine running.
- Checks for any codes that are stored in the ECM memory with engine off.
- Compares ECM's control data to typical data values.
- Checks to see if codes are intermittent problems.
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
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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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