Q Is it possible for an air-conditioning compressor to go bad so quickly? Our daughter bought a 2007 Nissan Altima with 72,000 miles last August from a dealership, and now the A/C doesn't work. She was told the compressor is dead. She'll get a second opinion, but I'm also wondering what your opinion might be.

Secondly, my husband drives a 2000 Jeep with 180,000 miles on it. A mechanic told him that to "save the life of his A/C" he should use the recirculate button. He said the A/C unit won't work as hard or cycle as much, and that'll keep him from having to deal with a similar problem that my daughter has. But I find it irritating because recirculated air gets stale and doesn't cool as efficiently in his car. I use the recirculate button only when we're following a stinky bus or truck. I can't seem to influence him on this, and if the A/C does die he'll blame it on not using the recirculate button, so it's a no-win situation. What's the real story?

A A marriage issue, perhaps? The Jeep is his vehicle, so why not let him operate it as he chooses? The recirculate button, by closing off outside air intake to the system, allows the system to cool more quickly and efficiently, particularly in high temperature-high humidity conditions. I'm not sure this would be a significant factor in compressor life, but it certainly won't damage anything.

The problem with your daughter's Nissan may not be the compressor itself. It may be the compressor clutch, engine management system or HVAC control module not signaling the compressor to engage. Most compressors that fail tend to make nasty noises when engaged, but low refrigerant pressure would prevent the system from allowing the compressor to engage.

Q I own a 1994 T-Bird with a V8 engine with automatic transmission and 146,000 miles. When the weather is hot, my car downshifts randomly either one or two gears momentarily and then upshifts back to overdrive. I can go months without a problem and then it can occur multiple times in the same day. There are no slippage problems with the transmission. Would you suspect an electronic problem?

A Ask yourself this question; what operational parameters would mandate a downshift? Factors would include a higher throttle setting, lower manifold vacuum, change in vehicle speed or a transmission in "limp mode." Check the sensors and electronics that communicate these parameters: throttle position sensor for a dead spot, manifold absolute pressure sensor for connections, contamination or intake air leaks, vehicle speed sensor for intermittent or inconsistent signal. If the transmission temperature sensor signals fluid overheating, the transmission will downshift into its limp mode.

If no electronic problems are identified, worn valve body or clutch components inside the transmission are a possibility.

Q I drive a four-cylinder 2008 Chevy Malibu with 31,000 miles on it. The car has a slow-speed miss. It does it when you are between 10 and 30 miles per hour. It does not do it above 30 mph. I've had it to the dealer twice for repair, but they can not find the trouble.

A With the sophisticated engine and transmission management systems on modern vehicles, I'm surprised the dealer did not find any diagnostic fault codes. Any consistent misfire should trigger and store a code.

My Alldata automotive database pulled up service bulletin 03-06-04-030G, dated April 2009, dealing with injector deposits and describing GM's fuel injector cleaning and testing procedure in detail.

Here's an idea. Find a shop with a chassis dynamometer and run the car in gear while it's connected to an electronic engine analyzer. If they can duplicate the scenario and experience the misfire, the analyzer should pinpoint which cylinder and the probable cause.

And don't overlook the possibility of the transmission torque converter clutch slipping, creating a slight shudder similar to a misfire.

Motoring note

You've still got Saturday and Sunday to visit the State Fairgrounds and check out the Minnesota Street Rod Association's 39th annual "Back to the '50s" celebration, with more than 12,000 pre-1964 street rods, hot rods, muscle cars and collector cars and trucks. It's automotive heaven for gearheads like me and a wonderful opportunity to relive America's automotive history.