Q: I have a 2000 Cadillac Eldorado ETC with 70,700 miles. The car is in excellent condition but there is an intermittent problem with both the air conditioning and heating systems. When the A/C is on, all of a sudden the blower motor shuts off and nothing comes out of the vents. A short while later, everything starts up again with the A/C working correctly. The same thing with the heater: The blower motor will be providing heat as normal, and then the motor shuts down with nothing happening. After a while the heater motor starts up again and works correctly.

When working correctly, the A/C puts out cold air and the heater puts out heat. I'm not sure if the same motor is used for both A/C and heat, but if so, then something is turning the motor off and on intermittently. I haven't taken the car to a repair shop because every time I try, everything starts working. Any ideas?

A: Your intuition is correct: There's just one blower motor to provide all front heating, ventilation, and cooling. It's located at the right rear corner of the engine compartment, within the firewall, partially obscured by the engine's right-bank camshaft cover. Your Eldorado's electronic climate control system has great on-board diagnostics, but it doesn't look at the blower circuit.

Fortunately, the blower motor, which contains an integrated speed controller, is easy to diagnose using traditional methods. I'd start by using the system until the blower stops. Then with key on, engine off and system on, give the blower motor a smack with the end of a hammer handle or similar hard object. If it springs to life, a fault internal to the unit is a sure thing. If this fails to do the trick, try wiggling the 3-wire connector leading to it; a change in operation indicates a connector fault.

If neither of the above tricks works, a simple check for power, ground and a variable voltage signal at the three connector pins will confirm or condemn the needed external elements for the blower motor to work. My hunch is the blower unit will be the culprit, or a poor connection elsewhere is inhibiting either power or ground from making it to the connector. Communication signals, such as the one between the heater and A/C programmer and the motor's speed controller, and the smarts originating the signal, rarely give trouble.

Q: My oil change monitor said it was OK to go 8,300 miles before an oil change was needed. Is this really OK? My dad is a 3,000-mile-oil-change kind of guy, and he'd kill me if he found out I was going this far.

A: Yes, it's OK, but I'd still look at the dipstick regularly for oil quantity and any visual signs of trouble - honey to brown is good, very dark is not so good, blobby consistency or milky color requires immediate attention. Modern engines run cleaner internally than those of the past, thanks to improved crank-case ventilation and more precise air-fuel ratio control, and improved motor oil and gasoline helps further. Your oil-life system looks at the way you drive, such as cold start temperatures, run times, speed, and load, to work out a smart estimate of remaining oil life, but in most cases doesn't physically analyze oil condition.

If it brings family harmony, perhaps you could change your oil at 25 percent life remaining.

Readers may send Brad Bergholdt email at under-the-hood@earthlink.net; he cannot make personal replies.