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Dear Jim,

I have a 1998 Nissan Sentra E (1.6 L, L4) that's been hesitating and running pretty rough for at least the last 2,000 miles, and I can't figure out why.

About 500 miles ago, I changed the oil and filters and had the car tuned up, but the idle still isn't right. A friend of mine suggested it might be a bad oxygen sensor, but I have no idea if that's true.

What can I do to find out? If it is a bad oxygen sensor, how expensive is it to replace?

-- Pete Smithson

Oxygen sensors can be a tricky little devils, bro.

They basically look like spark plugs, and yet I've seen them cost as much as $500 on some foreign cars. A new sensor for your Sentra shouldn't cost more than $70 or $80, but that's still cash you don't want to spend if you don't have to.

That said, of all the sensors hooked up to your car's central computer, the oxygen sensor is the one most likely to go out.

A little background: The oxygen sensor has only one job, and that's to monitor exhaust gas and tell the computer how lean or rich the fuel mixture is.

Once the computer knows this, it can more easily find a fuel-to-air ratio that will give you the most power with the least amount of fuel waste.

That's great, but the oxygen sensor is located right there in the car's exhaust system, which is a very hot place for any sensitive component to work for very long. In my experience, they tend to break down about every 35,000 miles. When that happens, you'll often get exactly the kind of symptoms you're describing.

Anyway, the only real way to find out is by testing the oxygen sensor, and that's not an easy thing to do yourself.

Take your Nissan to the shop and have them use their diagnostic computer. The testing itself shouldn't cost too much, and the computers they use these days are rarely wrong.

If it's an oxygen sensor at fault here, you'll know. If it's not, bro, then start thinking about the fuel injectors -- when they get clogged, the same rough idling can result.

Tip of the Week

Keep that coolant level up during these summer months, drivers, but don't stop there -- if you haven't had the whole cooling system flushed out recently, do it now.

Buffalo Jim, owner of Allstate Auto & Marine Repair in Las Vegas, writes a weekly column. Send your automotive questions to him at buffalojim@lasvegasniftynickel.com. If Jim picks your question to answer in Nuts & Bolts, you'll also be the proud recipient of a deck of official Buffalo Jim playing cards, presented in a classy, tin box bearing his hairy image.