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I took my car in to the shop because the "check engine" light was on and found out this was simply due to my gas cap being loose. Why would that set off the warning light? – Louis in Duncan, B.C.

Unless you're driving a vintage model, a loose or missing filler cap on your vehicle fuel system will trigger a "check engine" or "fuel door open" light.

Among the many causes of a dash-light warning, you've managed to get off easy.

"There's a system that sucks the vapour out of the tank and puts it into the engine under most driving conditions," says Glen Osborne, co-owner of Major Service in Langley, B.C. "And if there's a leak in the system whatsoever, the computer will sense it, and 95 per cent of the time it's the gas cap. But there's also vacuum lines that run up to the engine; if one of those has a hole in it, that'll set it off. And there's a purge valve and a vent valve and, if either one of those is defective, it will sense that too."

The gas cap plays an anti-pollution role. Along with the development of emissions regulations in Canada, since the early 1970s, vehicle fuel systems are closed.

If your cap is loose or missing, hydrocarbons are venting into the atmosphere.

In addition to conserving fuel, keeping the gas cap tightened may extend the life of the fuel pump – which is usually found inside the tank.

"With a lot of new cars, the fuel pumps are in the tank; that way if the cars are in an accident they're not exposed to flames or explosions," says Mike Salkus, owner of Speedy Auto Service in Victoria. "And the pumps work on pressure. It's very common for people to leave their gas caps loose, and that will set off the light because it's not building up any pressure in the tank. It's what we call vapour loss or leak. And that pressure helps prolong the life of the fuel pumps because there's pressure and the pumps aren't having to pump as hard. So, it's very important to make sure that pressure is built up in there, to keep the pump fully working at its capacity without having to overwork and burn out."

The "click" feature of a modern gas cap indicates when it's tight, and prevents over-tightening and damage. By keeping the cap fastened correctly, you can eliminate the worry and potential cost of an engine-warning light.

"When I have a customer phone me up who has had their engine light come on, the first thing I'll suggest to them is to go out and turn the gas cap tight, make sure it clicks a few times, then take the vehicle for a road test and see if it'll go out," says Salkus.

"If it doesn't, we need to go further into it and see what's wrong. But I always recommend they check their gas cap before they come in and pay me $60 to tell them it's loose."

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