Check engine light on: can I keep driving?

Is the light steady or flashing?

Check that first: the check engine light (officially the malfunction indicator lamp, or MIL) has two modes, steady and flashing, and they mean very different things.

On any 1996 or newer car or light truck sold in the US, the light is part of the on-board diagnostics (OBD-II) system, which watches the engine and emissions controls and stores a trouble code when something goes wrong (EPA).

Can I keep driving with a steady check engine light?

Usually yes, gently, if the car drives normally: a steady light is a request for service, not an order to stop. The EPA's own OBD fact sheet says the light "is intended to inform the driver of the need for service, NOT of the need to stop the vehicle" (EPA, 1997).

That doesn't mean ignore it. GM's owner's manual for the 2025 Chevrolet Equinox warns that driving continually with the light on can mean the emission controls work less well, fuel economy drops, the car may not run smoothly, and repairs can get costly. The EPA notes the same fault can be anything from a loose gas cap to a severe misfire.

Things to check yourself first:

1. The gas cap. A cap that wasn't tightened after refueling lets fuel vapor escape, and the OBD system can detect that leak (EPA). Tighten it and give it a few drives. See P0455 and P0442 (EVAP leak) for the gas cap codes.
2. Recent fuel. GM notes poor-quality fuel can cause poor running and turn the light on, and that it may take a full tank of good fuel to clear.
3. Anything that changed. GM also lists modifications to the engine, exhaust, intake or fuel system, and non-original tire sizes, as possible causes.

If the fault fixes itself, the light goes out on its own: under California's OBD-II regulation, the MIL is turned off after three consecutive driving cycles in which the fault isn't detected (13 CCR § 1968.2; misfire and a few other faults have their own rules). EPA's OBD rules for 2017 and newer vehicles (40 CFR § 86.1806-17) adopt that California regulation.

Then get the code read. The code says which system the fault is in; the light alone doesn't. See pending, stored and permanent trouble codes for what you'll see when you read them. Codes we cover in detail include P0420 (catalytic converter efficiency), P0171 (running lean), P0101 (mass airflow sensor), P0128 (coolant below thermostat temperature) and U0100 (lost communication with the engine computer). Any standard scan tool can read the code; you don't need a subscription.

What should I do if the check engine light is flashing?

Slow down, avoid hard acceleration and get off the road soon, gently: a flashing light means a misfire severe enough to damage the catalytic converter. For the misfire codes themselves, see P0300 (random or multiple misfire).

Under California's OBD-II regulation, the light blinks while the engine is misfiring at a rate the manufacturer has determined would heat the catalytic converter enough to damage it (13 CCR § 1968.2, section (e)(3)). The EPA calls it "a rather severe level of engine misfire" and says to "reduce speed and seek service as soon as possible" (EPA, 1997).

Manufacturers are more specific, and stricter:

• GM (2025 Equinox): a malfunction has been detected that "could damage the emission control system." Reduce speed, avoid hard acceleration and uphill grades, and reduce any trailer load. If it keeps flashing, park somewhere safe, turn the vehicle off, wait at least 10 seconds and restart. If it still flashes, see a dealer as soon as possible.
• Honda (2019 Fit owner's guide): a blinking light means a misfire has been detected. "Stop in a safe place and wait for the engine to cool down."

Where sources disagree. An older EPA consumer sheet (2000) says that with a blinking light you "can still drive safely but should minimize driving," avoiding high speed and heavy loads. The manufacturers' current manuals lean toward stopping. Our advice (inferred): follow your own owner's manual, and when in doubt, treat a flashing light as "get off the road soon, gently," not "finish the trip." If the car is shaking, losing power or smells of fuel, stop now.

Why the urgency: the catalytic converter already runs very hot. Ford's owner's manual warns that the emission system heats the exhaust enough to create a fire risk if you park, idle or drive over dry grass (Ford). Unburned fuel from a misfiring cylinder burns in the converter and adds to that heat, which is how a misfire damages it (our explanation, consistent with the regulation's catalyst-temperature basis above).

Which symptoms mean pull over now?

The signs in the table below matter more than any trouble code. Most of these mean find a safe place to stop, whatever the check engine light is doing; the last column says what the manuals advise for each.

Will a check engine light fail an emissions test?

In states that test with OBD, a lit check engine light usually means a fail. The federal test procedure in 40 CFR § 85.2222 reads the stored codes whenever the car's computer is commanding the light on and fails the vehicle if the light is commanded on. Rules differ by state; we checked the federal procedure and California's. Check your own state's program.

Don't clear the code right before a test. Clearing resets the readiness monitors, and the station will turn you away as "not ready." On most 2010–2011 and all 2012 and newer vehicles a permanent code also stays behind. See why your car says "not ready".