Check engine light on: what to do, and when you really must stop

Steady or flashing, that's the whole question

Carmakers use the two states deliberately. Steady means a fault is recorded and the engine is still running inside safe limits. Flashing means a misfire is happening right now and raw fuel is reaching the exhaust, where it burns in the catalytic converter instead of the cylinder.

A converter is one of the more expensive parts under a car. The difference between the two lights is often the difference between a sensor and a converter.

Start with the fuel cap

The evaporative system is sealed and monitored. A cap that was not tightened, or whose seal has hardened, lets the system detect a leak and light the dash. It's the single most common harmless cause.

Tighten it until it clicks and drive normally. If that was the cause, the light usually clears itself after a few driving cycles.

Why a code isn't a diagnosis

A scanner gives a code, and a code names the circuit or the reading that fell out of range. It doesn't name the broken part. P0420, catalyst efficiency below threshold, is the classic example: it is read as buy a converter, when an upstream sensor or an exhaust leak ahead of it produces the same code.

That's the gap between a free parts store scan and a diagnosis. The scan takes two minutes. Confirming which part actually failed is the work you're paying for, and it's what stops you buying a converter you didn't need.

What it costs to ignore it

A steady light left alone for months tends to get more expensive, not less. A failing sensor makes the engine run rich, which washes the cylinder walls, loads the converter, and quietly costs fuel the whole time.

There's also an inspection angle: a car with a live fault will not pass a pre-purchase inspection cleanly, and a buyer will price the unknown higher than the repair.

Frequently asked questions