Q: I have a 1998 Cadillac Deville with Northstar engine at 130,000 miles. Last summer, I noticed the A/C was blowing warm air, so I took it to the dealer. They charged the system and it worked great all last summer. This summer, the same thing happened, but they told me I needed a new compressor or condenser at a cost of $675. But I noticed that at highway speeds of 55 to 60 mph, it blows relatively cool air. But at low speeds and at idle such as at stop signs or red lights, it blows very warm air. If the compressor or condenser is bad, how does it blow cool air at highway speeds?

This has been a great car, with all the regular recommended maintenance. Great gas mileage for a full size car and with front wheel drive and traction engagement, it is great for our Pennsylvania winters. We have had a few 90-degree days that are brutal. I am not a mechanic. What are your thoughts on how to proceed?

A: It sounds like the Deville has a small refrigerant leak that may have gotten worse, or the technician that recently serviced the car is a bit fussier than the previous one regarding what's considered tolerable leakage.

An air conditioning compressor may require replacement due to internal mechanical problems or refrigerant leakage from its shaft seal or housing. Since your A/C works OK at road speed, I'm guessing the compressor is OK mechanically but may have a leaking seal or housing o-ring.

The condenser is a large, flat aluminum heat exchanger that's mounted in front of the radiator. These can develop refrigerant leakage due to mechanical injury or vibration-induced cracking of tubes, fittings or mounting tab attachments.

It's a shame the dealer's service adviser didn't communicate a little more clearly as to exactly which part or parts needed replacement and why. The price you were quoted sounds about right for one of these parts to be renewed, but too low for both. Diagnosing small leaks can be tricky. Any detectable leakage from an injured component justifies immediate repair, but a tiny amount of leakage from the compressor shaft seal or a fitting o-ring may be considered tolerable. If the Deville will run for a year between refrigerant recharging - and the leakage hasn't suddenly worsened - doing so again sounds reasonable.

The reason the Deville cools somewhat at road speed but not at idle is likely due to higher compressor RPM and increased airflow across the condenser. This indicates also that the system has leaked perhaps only about half of its 32 ounces of refrigerant.

I'd check back with the dealer for clarification on why the repairs were recommended. Perhaps they could pull the hard copy of the repair order, which would contain the technician's exact findings, and explain this to you, as opposed to the abbreviated version re-written by someone else on the soft copy given to you. Even though the EPA does not require leakage repair prior to recharging, as some localities might, it's certainly a best practice to reduce leakage on every vehicle as much as is reasonably possible.

It's best to evacuate and recharge an A/C system with the specified quantity of refrigerant, rather than simply top it off with a can or two purchased from the auto parts store. It's impossible to determine how much refrigerant is remaining, and how much to add. Most systems are very fussy about being charged to the precise recommended capacity.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.