Q You often talk about timing belts vs. timing chains. How can I tell which my car has, and when I should change it? We have a 2002 Nissan Altima six-cylinder with 106,000 miles, a 2005 Hyundai Elantra four-cylinder with 80,000 miles and a 2006 Hyundai Tucson four-cylinder with 15,000 miles. I'm sure the '06 is good for a while, but what about the others?

A Your first step is to check the owner's manuals for service requirements and maintenance schedules. Is the crisp, new $100 bill still folded neatly between the pages? Most owners don't read their owner's manual, so that $100 bill is still there, waiting for you. Just kidding, of course, but it's a not-so-subtle reminder of the value in reading the manual.

If you no longer have the owner's manual or you bought the vehicle used and the manual wasn't included, check with your public library to see if they offer online access to Alldata, an automotive database that includes maintenance and repair information for virtually every car built after 1981. You can also purchase online access to Alldata for your particular vehicle. You'll find all the details at alldatadiy.com.

Your Nissan with the 3.5-liter V6 features timing-chain-driven cams that require no routine service. Your Elantra and Tucson use a timing belt that should be inspected at 30,000-mile intervals and replaced every 60,000 miles or eight months. Both are interference engines, meaning a timing-belt failure could lead to major engine damage.

Q I have a 2001 Audi TT that I bought new. A couple of weeks ago, the check-engine light came on. The dealer indicated the catalytic converter was bad and would need to be replaced for $900. Is it unusual for the catalytic converter to fail? Could there be an engine problem that caused this? The dealer reset the light, and it has not come back on.

A Catalytic converter failure isn't unusual, but it's not particularly common. And in most cases, the failure is not total -- the fault code that triggered the check-engine light often describes a loss of catalytic efficiency.

Typically, there are two potential causes for this. First, physical damage. If the converter takes a hit from road debris or if the converter is grossly overheated by an excessively rich fuel-air mixture, the catalyst material inside the converter -- typically platinum or palladium -- can crumble and restrict exhaust flow.

The second and primary cause for efficiency loss in a converter is exposure to unburned fuel in the exhaust. An excessively rich fuel-air mixture in the short run, or normal mixtures over the long run, can eventually coat or plate the catalytic material with carbon and exhaust residue, reducing efficiency.

The front oxygen sensor measures the percentage of oxygen in the exhaust before reaching the converter, and the rear oxygen sensor monitors the converter's efficiency in catalyzing and oxidizing unburned fuel. When efficiency drops below a certain threshold, it will trigger the check-engine light for the rear oxygen sensor. This is probably what happened to your Audi. Since the light has not come back on since the fault code was cleared, you can drive the car without fear of harm.

Some repair shops offer a catalytic converter "cleaning" system that may help restore converter efficiency. And finally, although your car is beyond its emissions warranty, remember that the computer and catalytic converter are covered for eight years or 80,000 miles, whichever comes first.