Dear Jim,

For the last month, I've been noticing sluggish performance from my 2001 Chevy Blazer (4.3 L, V6). The "check engine" light came on last week, so I took it to a shop where they told me I had a bad mass airflow sensor.

The mechanic replaced the sensor, but that didn't help. Then he replaced the spark plugs, oxygen sensor, distributor cap and rotor, and the truck still died a few blocks from the shop.

Now I've gone to a new mechanic, who says it's the mass airflow sensor.

What's the problem here, Jim? Both mechanics' computers say it's the part I just replaced, but I don't want to waste more money fixing a sensor that's probably fine.

-- John Tolley

One problem, bro, is that mechanics often put too much faith in computer diagnostics. Vehicle computers are extremely useful gadgets, sure, but they can only tell us when a system isn't working right -- not necessarily what's actually wrong with the car.

The computer might be indicating a bad mass airflow sensor in your Chevy, but replacing it (along with a bunch of other good parts) didn't help, so now you know it's not the sensor itself that's making trouble.

Think about what the computer is really telling you. If it says "mass airflow sensor," this might just mean there's not enough air flowing past the sensor before it gets to the fuel system. Without enough air, the engine suffocates before it even has a chance to get going. Some other faulty part is making this happen, not the sensor.

Your timing chain could be the culprit. If it's out of position, engine timing gets thrown off and airflow gets restricted, causing the sluggishness and stalling you're talking about. A compression check on the engine will tell you whether this is the case.

You also could be looking at some kind of exhaust system blockage, such as a plugged catalytic converter. You can test this by disconnecting the system at the exhaust flange for a moment and tying it off to the side. Your truck will be loud, but if it suddenly runs well, you've found the problem.

Anyway, bro, car computers can tell us a lot these days, just not everything. But who knows? Maybe in the future they'll be able to diagnose mechanics.

Tip of the Week:

That slightly dirty air filter they like to show you during oil changes is still good, drivers. Once it starts looking really gray, it's done.

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.