The CEL. (That’s what mechanics call the “check engine light.”) The folks over at Lifehacker recently posted a list of “the five most common causes of a check engine light and what you should do about them,” and it’s not a bad list. Check out the article if you wish, but I’ll save you some trouble. The list: faulty oxygen sensor, loose or faulty gas cap, faulty catalytic converter, faulty mass airflow sensor, bad spark plugs and/or wires. They suggest taking your car in for diagnostics (which will work, of course), but most Toolmongers know by now that with a simple reader you can at least see what the computer thinks is wrong. And if you don’t own a reader, you can generally rent one (or borrow one with the purchase price as security) from auto parts stores.
A few additional comments beyond Lifehackers, though, from the experience of one who’s suffered through most of these failures:
Replacing an oxygen sensor is generally dirt simple, usually involving just unscrewing the old one and replacing it with the new one. They’re sometimes a little difficult to get at, and you want to be careful about banging the replacement around when installing it. Otherwise, it’s no big deal. Definitely not worth paying a couple of hours of book rate at the shop if you’re tight on cash. Just make sure you get the correct sensor when you order the replacement. Hint: Take the old one with you when you go to pick up the new one, if possible. If not, snap a camera phone photo of the old one.
Yes, a leaky gas cap will indeed screw you over in emissions testing. If you have to replace yours, there’s nothing wrong with buying a cheap-ass version from the auto parts shop, but make sure it’ll fit under the door flap before you purchase. And just skip any locking version, or permanently unlock it and ditch the key. You’ll lose it eventually and get stranded.
In terms of replacing the catalytic converter, be sure to check your warranty first before you head down to the local Midas. Some manufacturers offer extended warranties (as much as 60,000 miles) on converters, so you might be entitled to a free (or cheap) one at the dealership. If not, you might (depending on where you live) have some options at the local muffler shop which will save you over OEM. Just be sure you’re compliant with local emissions standards.
Mass airflow sensors are usually a bolt-off, bolt-on replacement. Don’t spend $200 just to have someone turn a few nuts for you.
The Five Most Common Causes of a CEL [Lifehacker]