Headlights an on-again, off-again affair

Q. One of the four HVAC vent covers on my Nissan's dashboard does not open or close properly. I'm told that it cannot be fixed without replacing the entire dash. Do you know where I could purchase the vent covers? Also, the strip of plastic on the shifter that lists the gears is cracked. Again, I'm told this item isn't available separately and that I'd have to replace the whole console. My car is only five years old! I can't get parts?

A. You're backed into a corner here, because there isn't a part number for the pieces you need. The vents and the bezel for the shifter are available only as part of a larger assembly. That's true for many items you might need to replace, such as the rubber bushings for sway-bar links, the ones that come only in a box as a compete assembly.

Owners of older, more collectible cars often can find these types of parts in the restoration aftermarket. Because your vehicle is so new, however, you probably won't be able to find any aftermarket or new/old stock parts. You'll need to look in a salvage lot for used parts.

Q. I need help troubleshooting my 1996 GMC Safari. I replaced a bunch of ignition parts, and my scanner gave me code 300. It has four oxygen sensors. I cleaned one sensor that was reading "low voltage," but the truck still idles rough. The engine now surges at stoplights, and brownish-gray smoke comes out of the exhaust when you floor it.

A. One bit of advice: Never clean an oxygen sensor. If it won't clean itself, it's toast. These sensors are sitting out in a stream of ultrahot exhaust, and they have heaters onboard to boot. All you're likely to do, when you clean them, is to contaminate them.

Besides, one oxygen sensor isn't the cause of your misfire. Your P300 trouble code indicates a random misfire, not one from any particular cylinder.

Smoke coming out of the exhaust is caused by a rich mixture, and that could be the result of a bad mass airflow sensor, customarily referred to as a MAF sensor, or of high fuel pressure, a dirty air cleaner, an air leak, or a hundred other things. Best to have this looked at by a professional.

Q. Can a headlight bulb work and then sometimes not? My car's headlights work and then they quit, at random. I pulled the connection cable off, and it isn't broken. It does have some type of light-brown grease in there. I can put the cable back on and it will work, but then it stops again. What's going on?

A. Light bulbs generally fail electrically open, not shorted. The tungsten filament inside breaks, often in several pieces, and won't conduct electricity.

I've seen little pieces float around and sometimes make a connection, illuminating briefly. But the filament is invariably shorter, lower-resistance and very, very bright, burning out completely within a minute or two. Usually this will blow the fuse, not simply make the bulb go out.

So you've got a loose connection, probably in the socket. That brown goo is grease of some sort, applied at the manufacturing plant to keep moisture out of the connector to prevent corrosion.

Headlamps run hot. I've seen the plastic connectors warp enough to allow metal parts inside to shift, giving a poor connection. Pull the plastic connector free of the back of the bulb, and use a small screwdriver, inserted next to the female spade lug, to depress the tang that holds the connector into the plastic block. Then you can pull it out the back of the plastic block.

Squeeze the metal slightly with a pair of pliers to improve contact, but not so tightly that you can't get the male connector on the bulb to go in, and reassemble the whole mess. While you're at it, dab on a little dielectric grease to keep the water out.

Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com.