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Glossary / Malfunction Indicator Lamp (MIL)

Malfunction Indicator Lamp (MIL)

The dashboard lamp — usually labelled Check Engine — that the OBD-II system switches on when it confirms a fault.

Also called: MIL · check engine light · service engine soon

What it means

Definition

CARB states that if a problem or malfunction is detected, the OBD II system illuminates a warning light on the vehicle instrument panel to alert the driver. The lamp reports that a code was confirmed; it does not report what the fault is. Federal test rules treat a missing lamp as a failure in itself: a vehicle fails the OBD test if the lamp is commanded to be illuminated and it is not visually illuminated according to visual inspection.

Why a buyer cares

What it changes

The lamp being dark is not the same as the computer being clear. That is why an inspection compares what the computer commanded against what the cluster actually shows, and why a scan says more than a glance at the dash.

In practice

Where you meet it

The instrument cluster, where it lights briefly at key-on as a bulb check, and as a pass or fail line on an emissions inspection report.

Two things that look alike

Not the same as

The lamp is the signal; the diagnostic trouble code is the stored detail behind it.

Read next

Related terms

Diagnostic Trouble Code (DTC)

The code an OBD-II system stores to identify which monitored system it found a fault in.

OBD-II (On-Board Diagnostics II)

The standardised self-diagnostic system in modern cars, read through a connector under the dash.

Limp mode

The common name for a protective state in which a control module limits engine power after a fault.

Emissions inspection

A state or local test of a vehicle's emissions performance, often required to register it.

Where this comes from

Sources