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Short Summary
Main recommendation: If the check engine light is on steadily, you can usually keep driving, but if it is flashing, pull over and have the vehicle towed, and in either case get it scanned to find the cause before clearing the light. The light signals a fault in the engine, transmission, traction control, ABS, or four-wheel-drive systems, and clearing it without a repair usually means it returns.
Key Points
- A check engine light has many names, including MIL (malfunction indicator light), DTC (diagnostic trouble code) light, SES (service engine soon) light, and CEL (check engine light), and its icon varies by vehicle.
- It is not the same as a maintenance or service light, which may appear as a wrench or oil can and only means an oil change or routine service is due.
- A light does not come on for no reason: vehicle computers monitor sensors and trigger the light when a reading is wrong, and causes can be as simple as a loose gas cap or low battery voltage.
- Low oil does not directly trigger the light, but very low oil can produce air bubbles in the oil pressure system or affect variable valve timing (VVT) and the timing chain, and changing the oil may not fix the resulting damage.
- A light that turns off on its own can happen when the monitored condition passes again, such as a temperature-dependent fault or a battery that regains voltage after sitting.
Details Worth Noting
- Concrete instructions:
- If the check engine light is steady (not flashing), it is acceptable to keep driving, but get the vehicle to a place where it can be scanned to find out why the light is on.
- If the check engine light is flashing, pull over and have the vehicle towed. Do not keep driving, because it could cause more damage and cost more money.
- Use a scan tool to read the diagnostic codes stored in the vehicle.
- Treat the code as a direction, not a final diagnosis; diagnose the condition further before replacing parts.
- Complete the repair first, then clear the light with the scan tool.
- Do not clear the light without diagnosing and fixing the problem, since it will most likely come back on.
- If you replaced a part, expect that the vehicle's readiness monitors need to run and pass before emissions testing.
- The speaker says that in states with emissions tests, the vehicle must pass its readiness monitors, so clearing the light with a scan tool just before an inspection will not pass it.
- Battery voltage problems can cause the light, and charging the battery may cancel it, as with a vehicle that sat for about a month.
- Newer engines, such as the Buick example shown, rely on VVT, which the speaker describes as very sensitive to oil level.
Bottom Line
The check engine light is a diagnostic signal rather than a simple on-off switch, so the practical goal is to identify the cause with a scan and repair it before the light is cleared or the vehicle is tested.