MIL (check engine light)
Workshop glossary › Diagnostics & protocols · UpdatedThe amber check-engine light. The ECU turns it on when an emissions-relevant DTC has been confirmed across two drive cycles.
The Malfunction Indicator Lamp (MIL) — better known as the check-engine light — is the dashboard symbol the ECU turns on when it has confirmed an emissions-related fault. Under OBD-II regulations the lamp must illuminate steady for most faults and flash for active misfire severe enough to damage the catalyst.
A solid MIL means "continue driving with caution and book service soon." A flashing MIL means "stop driving as soon as it is safe" — the ECU is detecting cylinder misfire bad enough that raw fuel is reaching the cat and is about to destroy it. Workshops should always ask the customer which one they saw on the way in.
The MIL itself can be cleared by clearing the associated DTCs, but the ECU will re-illuminate it on the next drive cycle if the fault recurs. Permanent DTCs (introduced under CARB and adopted in EOBD from 2017) will keep the MIL on regardless of clearing until the ECU self-verifies the repair.