The Ride-Along Mechanic

Cars built since the mid-late 1990s have a kind of ride-along mechanic in the form of the On-Board Diagnostics (OBD) computer which not only runs the show but lets you know when something’s not right via the “check engine” light that comes on in the dashboard.

More specifically, stays on.

The “check engine” light should come on — briefly — when you first start the engine. But it should go out once the engine is running. If it stays on while the engine is running, then something’s not right. Probably with the emissions control system but it can be other things, too.

This is a smart light, as opposed to the idiot lights pre-OBD cars came with — which lit up to let you know it was already too late. For example, the “oil” light would come on when you’d already lost almost all oil pressure; the “amp” or “gen” light would let you know the alternator (or generator) was no longer making electricity. Very soon — once the battery ran out of juice — you’d be in the market for a tow truck.

They didn’t give you much warning a problem was developing.

Gauges were an improvement — if you paid attention to them and if you understood what a normal vs. an abnormal reading was. This was the big problem with gauges. They didn’t light up. Ever. Which meant that if you weren’t scanning them regularly as you drove, you could lose oil pressure or drive around with the engine temperature in the red zone and be oblivious to these potentially catastrophic problems — until they became actually catastrophic.

Then, of course, you’d know.

The check engine light — which is really the Malfunction Indicator Light — lets you know more. And — usually — in plenty of time to deal with whatever’s wrong before it becomes a bigger problem.

If you have a scan tool.

Most people don’t.

Aye, there’s the rub.

Without the tool, all you’ve got is “check engine.” And that could mean anything from a loose gas cap (most new cars are designed to prevent gas vapors from escaping into the environment; if you didn’t completely tighten the gas cap, a fault is registered and the MIL lights up) to something more serious, such as a problem with an oxygen sensor or fuel injector which could — if left unfixed — ruin an expensive catalytic converter.

Or two of them.

Most new cars have at least that many; several have four of them. They are not cheap.

If you have a scan tool, you can access the trouble codes — which in turn either tell you what the problem is or point you in the right direction. The cheapest/most basic scan tools — which cost about $60 and can be bought at any auto parts store — only give you the code but the better models — which aren’t obnoxiously expensive, around $120 — will also tell you what the codes mean.

With any scan tool, you can also re-set the MIL — turn off the “check engine” light by clearing the stored codes.

If you haven’t fixed whatever caused the light to come on, the light will come back on — and FYI, smog test stations are aware of this trick — so it won’t work as far as getting your car passed if there’s some still-there issue with the emissions control system. But it will work if you fix the problem — a loose gas cap, for instance — and without having to go to a shop for assistance. Given the high cost of shop rates, the cost of the scan tool is money well spent if it saves you from going to the shop even once for just about anything that involves them taking a look at your car for you.

Not to mention the time saved.

Also, knowledge is power. Even if you don’t work on cars, having some idea what the problem is before you take it in is always a good idea. Especially if you get a diagnosis that doesn’t jibe with what the code reader told you. It’s like having a paint depth reader when going to check out a used car. You don’t have to be a body/paint man to use the tool — and avoid buying a car that was wrecked and slathered with body filler.

Like the paint depth tool, anyone can use the scan tool. All OBD-equipped cars have the same universal OBD port, usually located in the driver’s side footwell area. It’s a pretty obvious electrical plug in. It doesn’t matter what make/model car you have. All OBD scan tools fit all OBD-equipped cars.

Instructions will come with the tool, but — in general — you plug the tool in to the OBD port, then start the engine and read the codes on the tool’s LCD display. The instructions will also tell you how to clear the codes and re-set (turn off) the “check engine” light.

It’s a tool worth having, even if you’re not very handy!


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