Common Cause of Pressure Loss is Also The Simplest to Fix

Dear Car Talk:

My 2013 Subaru Outback recently freaked me out! Just out of the driveway, I noticed the "check engine" light was on, and the brake and cruise control lights were flashing. I eventually Googled these symptoms and the recommended fix was to turn the engine on, take off the gas cap, put the cap back on, then turn off the engine.

I did this and hocus pocus! When I restarted the engine, the demon was gone! All the warning lights were off. Does this make sense to you? -- Frank

Total sense. And keep in mind, Frank, not much makes sense to me.

One of the check engine light's duties is to alert you when there's a problem with your emissions system. And part of your emission controls is a sealed fuel system. That traps gas fumes and keeps them from leaking into the atmosphere. The whole fuel system is kept under pressure. If it loses pressure, your car's computer concludes -- correctly -- that there's a leak somewhere, and it turns on your check engine light.

One of the most common causes of a pressure loss in the fuel system is also the simplest: a bad gas cap or one that wasn't put on correctly.

So, if on your last trip to Bubba's Tacos and Gas, Bubba was checking his GameStop stock while he was pumping and didn't thread the gas cap correctly, that could cause your fuel system to slowly lose pressure. Once it lost enough pressure, the computer said "Leak!" and turned on your check engine light to alert you.

Sometimes, when there's a fuel system fault, the computer will also temporarily disable other systems -- like your cruise control -- to be safe. That's why those other warning lights came on at the same time. If removing and replacing the gas cap turned off the warning lights, do your happy dance and keep driving. You're probably all set.

If the check engine light comes on again, though, you'll need to have the car scanned. When that light comes on, the computer stores a "fault code" that tells your mechanic why, so he knows where to look for your problem. If your next scan turns up a code for a bad gas cap, you may just need a new one. Fortunately, Frank, I think the '22 gas caps are out now with 1.9% financing.


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