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.
Test T: Malfunction Indicator Light Always On: Description
Battery positive voltage is supplied directly to Malfunction Indicator Light (MIL). Powertrain Control Module (PCM) illuminates the MIL by grounding MIL control circuit.
MIL informs driver that a malfunction has occurred and vehicle should be taken in for service as soon as possible. MIL illuminates during a bulb test and a system test. A Diagnostic Trouble Code (DTC) will be stored if a MIL is requested by diagnostic test. MIL will illuminate with ignition switch in RUN position and engine not running. MIL will turn OFF when the engine is started. MIL will remain illuminated if self-diagnostic system has detected a malfunction. MIL may turn OFF if malfunction no longer exists. If MIL is illuminated and engine stalls, MIL will remain illuminated as long as ignition switch is in RUN position. If MIL is not illuminated and the engine stalls, MIL will not illuminate until ignition switch is cycled OFF, then ON.
NO RELATED
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.