Your check engine light is flashing. Pull over.

Steady light: book it in this week. Flashing light: pull over.

That's the whole answer, and most people don't know it. The two lights look nearly identical on the dash and they mean completely different things.

Why flashing is different

A flashing check engine light almost always means a misfire. One or more cylinders isn't burning its fuel, so raw fuel is going straight out the exhaust port and into the catalytic converter.

Converters run around 1,200°F normally. Dump unburnt fuel into one and it ignites inside, and the temperature climbs past what the honeycomb can survive. It melts. Not over months. Over minutes of driving.

So the misfire itself might be a $200 coil pack. Drive on it for twenty minutes and you've added a converter, which on most vehicles runs well over a thousand and on some trucks a great deal more.

What to do right now

• Get off the road somewhere safe.
• Turn it off. Don't idle in the parking lot "to see if it clears."
• Call us on (253) 375-7057, or call a tow.
• If you absolutely must move it, go the shortest possible distance at the lowest possible speed.

What causes it

The usual suspects, roughly in order of how often we see them:

• A failed ignition coil. Common, cheap, and the most likely answer on anything with over 100,000 miles.
• Worn spark plugs, especially if nobody's changed them and the mileage says they should have been.
• A failing fuel injector.
• A vacuum leak causing a lean misfire.
• Something mechanical — low compression, a burnt valve. Less common, more expensive.

The steady light

A steady light isn't an emergency. It also isn't nothing. It means the computer logged a fault, and the range runs from a gas cap that isn't sealing to an oxygen sensor to the early stage of something that will eventually leave you on the shoulder.

Drive it, but get it read. And read properly — the free scan at the parts counter gives you a code, not a diagnosis. We've written about why those two aren't the same thing.

One more thing

If the light came on and the car also started running rough, lost power, or made a noise it wasn't making before, treat that as the flashing case even if the light is steady. The light is one input. What the car is doing is a better one.