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.
On-Board Diagnostics: Introduction
The ECM has an on-board diagnostic system, which detects malfunctions related to engine sensors or actuators. See Fig 1. The ECM also records various emission-related diagnostic information. See Fig 2. The Malfunction Indicator Light (MIL) on the instrument panel illuminates when the same malfunction is detected in 2 consecutive trips (2 trip detection logic), or when the ECM enters fail-safe mode.
The MIL is located on the instrument panel. The MIL will light up when the ignition switch is turned to ON position without the engine running. This is a bulb check. If the MIL does not illuminate, see WARNING LIGHTS in appropriate INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. When the engine is started, the MIL should go off. If the MIL remains illuminated, the on-board diagnostic system has detected an engine system malfunction.
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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