Check engine light steady or flashing: what each one means and what to do

Turn the key to on, or push the start button, and most of the dash lights come on for a moment. The check engine light is one of them. The car is showing you the bulb works. If it goes out once the engine is running, there is no stored problem it wants to tell you about.

A lot of cars also have a separate maintenance or service reminder. That one tracks oil change timing and is a different thing. The check engine light is usually an outline of an engine in amber or yellow, sometimes with the word check.

A steady light means the engine computer saw a reading it didn't like and stored a trouble code. The cause can be small, or it can be the start of something bigger. Common ones are a loose or worn gas cap, an oxygen sensor, a small vacuum leak from a cracked hose or a tired intake gasket, a misfire that comes and goes, or a sensor sending a bad reading.

If the car drives the way it always has, you don't need to pull over. But don't leave it on for months either. A light that stays on hides the next problem, because when a second fault shows up the light is already lit and you get no new warning.

Treat a steady light as more urgent when other things come with it: rough running, a loss of power, a temperature gauge that climbs, a fuel smell or a new noise from the engine.

A flashing check engine light is the serious version of the same warning. On most cars it means one or more cylinders are misfiring badly. The car may shake at a stoplight, stumble when you accelerate or feel down on power.

Ease off and drive gently. Skip hard acceleration and highway speed if you can. If you are out on I-435 or the Kansas Turnpike when it starts flashing, get off at the next safe exit instead of pushing on to where you were headed. If the light keeps flashing or the car runs badly, find a safe place to stop and shut it off. Either way, get it looked at soon and keep it off long drives until it is.

A cylinder that misfires still gets fuel. It just doesn't burn it. That raw fuel goes out with the exhaust and reaches the catalytic converter, which is already hot from doing its job. The fuel burns inside the converter and can overheat it until the inside melts or breaks apart.

Once that happens the converter has to be replaced, and it is one of the pricier parts under a car. So a misfire that started small can turn into a much bigger bill if you keep driving on it.

A loose gas cap really can turn the light on. The fuel system is sealed, and the car runs its own leak check on it. A cap that wasn't tightened, has a cracked seal or was left hanging after a fill-up looks like a leak to the computer.

Tighten it until it clicks, if yours clicks, and drive normally. The light doesn't go off right away. The car has to run its check again over the next several drives. If the light stays on or comes back, either the cap wasn't the cause or the cap itself is worn out. A gas cap doesn't make the light flash.

Reading the code takes a scan tool plugged in under the dash. The code tells you which system set the light and what reading it didn't like. It doesn't always tell you which part failed. A code for a lean mixture, for example, can come from a vacuum leak, a dirty sensor, a weak fuel supply or a cracked intake hose. A code that says the camshaft and crankshaft readings don't line up can mean a sensor, or it can mean a timing chain that has stretched.

That is why Josh reads the codes and then checks what they point to before he quotes anything. Swapping whatever part the code description names is how people end up buying parts they didn't need. Some fixes land on his list, like gaskets, timing belts and chains, and camshafts. If yours turns out to be something he doesn't do, he will tell you that too.

Clearing a code without fixing the cause only turns the light off for a while. It comes back once the car sees the fault again, and clearing it also wipes out stored information that helps track the problem down.

A few other warning lights get mixed up with this one, and some of them are more urgent. If you can't tell which one is lit, text Josh a photo of the dash and he will tell you what he thinks.

• Red oil can or oil pressure light: pull over safely and shut the engine off. Low oil pressure can ruin an engine quickly.
• Red temperature light, or a gauge in the red: the engine is overheating. Pull over, shut it off, and never open a hot radiator cap.
• Battery light: the charging system has a problem, and the car is running on what is left in the battery.
• Red brake light: make sure the parking brake is all the way off. If it is, the brake fluid may be low, so get the brakes looked at before you drive much.