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ask joanne

My engine light always comes on. I've taken it to Honda several times when this happens and paid the $50 to have them "check the light with their computer" and then tell me if it's a big deal or not. The first time this happened I paid $300 to have the problem fixed, even though they said it wasn't a big deal but if I didn't fix it, my engine light would keep coming on and I wouldn't know if it was this problem or a different one. Despite this, the light keeps coming on. What could be causing this, and what should I do? - Janice in Moose Jaw

Assuming you don't want to follow the Red Green approach and use duct tape to cover the engine light, you'll need to find out what's causing this.

Since the 1980s, vehicles and engines have been increasingly controlled by computers. Dashboard warning lights and gauges were developed in passenger cars to alert drivers to potential issues, with the intention that they would then be fully diagnosed by qualified service technicians.

In this case, you've done this and paid $300 for the privilege, yet you're none the wiser. A Honda Canada representative I spoke with said an illuminated check-engine light can mean a number of different things, and therefore should be diagnosed by a mechanic. "It could be an O2 sensor, an emissions issue, or something more serious - so you really don't want to ignore it," says the rep. If you've continuously gone to the same dealership and they haven't been able to determine the problem, Honda recommends getting a second opinion.

So that's exactly what I did.

Here's what a 40-year veteran mechanic and import specialist had to say about warning lights: "Usually I find that an illuminated 'service engine soon' light has to do with the Evaporative Emissions Control System (EECS ) - and that has to do with the amount of pollutants coming out of the tailpipe. There are oxygen sensors screwed into the exhaust pipe which measure the amount of impurities in the exhaust gases. If you have, say, five parts of carbon per one million parts of oxygen, the light would be triggered to come on," says Russ Perry.

"With the EECS, it could also simply be a loose cap on your fuel tank, because the sensors can tell if there is pressure in the fuel tank. Another thing that can trigger the light is if you start burning different brands of fuel which all have varying additives, and the oxygen probe can sense that and cause the light to come on."

Your vehicle's mileage can make a difference when it comes to triggering the engine light. "When cars hit roughly 150,000 km, they consume oil. If you were burning a tablespoon of oil every week, you could effect the oxygen probe and the service engine light would come on. If your car is at, say, 300,000 km, you could go into the dealer every time the light comes on, but unless they rebuild the engine, it'll keep coming on," says Perry.

So, if the warning light seems to be malfunctioning, can you simply disconnect it? "The electronics are state of the art; the sensors are picking up a problem, and that's their job. People will ask me to cancel their light, and I say 'no', because we haven't addressed the problem. It's amazing; some people think they can just cancel the light and get on with their day," says Perry.

A friend with a VW Corrado was annoyed by what initially appeared to be an unnecessary check engine light. It turned out to be the failure of the head gasket, which in turn was pressurizing the internal components of the engine and putting it at risk of catastrophic failure. The situation was only discovered - and ultimately avoided - by taking the vehicle to a certified mechanic.

When trying to wade through automotive problems (especially for out-of-warranty vehicles), many motorists use On-Board Diagnostics (OBD), via an aftermarket handheld device or cable which attaches to your computer. These consumer devices cost around $100. If there's a problem with your vehicle, when plugged in, the OBD should bring up a fault code. You probably won't be able to fix the problem, but you can reference the code in the accompanying manual to find out what's wrong. At the very least, the OBD report provides an opportunity to get more information on your issue by searching automotive forums online.

Remember that it is always recommended to consult the owners' manual to learn about a vehicle's warning lights. Ultimately, though, with the computerization of vehicles these days, you really need to develop and rely on a good relationship with a mechanic you trust. If you think you're being ripped off, or feel things aren't working - as with any relationship - go with someone else.