Engine diagnostics and check engine lights at your home in Kansas City, Kansas

The engine computer watches sensors all through the engine, the fuel system and the exhaust. When a reading stays out of range long enough, it stores a code and turns on the light. That is all the light means. Something was out of range.

The list of things that can do it is long. A gas cap that didn't click shut is on it. A vacuum leak can do it too, and so can a sensor going bad, a misfire, a leaking gasket letting air in where it shouldn't, or a timing problem inside the engine. The light looks the same for all of them. Finding out which one it is takes a diagnostic.

How the light comes on tells you more than the fact that it came on. A steady light and a flashing light mean different things, and if the car is also shaking hard, it is better to leave it parked and have Josh come to it than to drive it across town.

• Steady: something needs a look. Drive gently, pay attention to how the car runs, and get it read soon.
• Flashing: usually a misfire bad enough to dump unburned fuel into the exhaust, which can cook the catalytic converter. Ease off, skip hard acceleration and get it looked at right away.
• Light plus a red warning like oil pressure or temperature: pull over somewhere safe and shut the engine off.

Many people have had a parts store read the code already. That is a fine first step. But the code says what the computer noticed, not what caused it. A code for a lean engine can come from a cracked vacuum hose, a leaking intake gasket, a dirty sensor or a fuel problem. Buy the part the code names and you may still have the light next week.

So Josh reads the codes first, and then he checks what they point to. That can mean looking for a split hose, testing a sensor, listening for a leak, checking which cylinder is missing, or checking the timing. Once he knows what is actually wrong, he tells you and gives you a clear quote for the fix.

Plenty of engine trouble never turns on a light, or turns it on late. If the car is doing any of the things below, it is worth a diagnostic even with no light on. Text Josh a short video if it is a noise. It helps a lot.

• A rough idle, a shake at stoplights, or a stumble when you pull away
• A ticking, knocking or rattling that comes from the engine itself
• Blue or white smoke out the tailpipe, or smoke from under the hood
• A burning oil smell, a sweet coolant smell, or a raw fuel smell
• Less power than it used to have, or worse gas mileage for no clear reason
• A rattle on cold starts that goes away once the engine warms up

A lot of what a diagnostic turns up is work Josh does himself: a leaking gasket, a worn belt, pulley or tensioner, a radiator or hose, an alternator or starter, or trouble with the timing belt, timing chain or camshaft. If the cause is one of those, you get the quote and decide. Say yes and the repair can happen at your house too.

Some engine jobs are bigger than a driveway allows and need a shop and a lift. If yours is one, Josh says so up front, so you aren't left guessing.

A cheap code reader can turn the light off. It does nothing to whatever turned it on, and the light comes back once the computer sees the problem again. Clearing it also wipes some of the stored details that help find the cause. If you can, leave the light on until Josh has read it.

Call or text Josh at (913) 256-5056 with the year, make and model, whether the light is steady or flashing, and what the car is doing. If a parts store already read the code, send that along too. Engine Doctor is based in Kansas City, Kansas. If you are somewhere else in the metro, ask if he comes out your way.