Is there a way to know if my air conditioning needs service? The repair shop I frequent recommends an annual checkup, and since my A/C seems to work fine I’m wondering if this is really necessary. Could I cause damage by not servicing it? What’s done during a service anyway?

– Nick Russo, Gilroy, Calif.

Good question, Nick, and this is the time of year to tackle it. I’m certainly one for a periodic inspection and for keeping up with needed maintenance, but an annual A/C checkup might do more for the shop’s bottom line than your vehicle.

Let’s look at some quick and easy things one can do to see if service is needed.

First, an outlet register temperature test should be performed. An instant-read cooking thermometer works well for this. Insert the thermometer in the center air vent and take the vehicle for a drive, using the A/C and medium blower speed. In dry climate areas, the A/C should cool to about 30 to 40 degrees less than the outside air temperature. In humid areas it should cool to perhaps 15 to 30 degrees less.

Let’s say that the system cools, but not as well as desired while driving at speed. Refrigerant loss, due to inevitable seepage past seals or faulty hose fittings, is a common cause of reduced performance. After a gradual decline, the refrigeration system will eventually shut down completely, blowing only warm air. Diagnosing and repairing a leak is a pro-level repair job due to the tools and techniques required.

Another, but less likely, cause for insufficient cooling is a fault in the ventilation system. Sometimes warm air is accidentally metered with the cool air, increasing outlet temperature. If the system cools OK while driving, but declines in performance while at a stop, the condenser cooling fan might not be working, or condenser airflow might be reduced due to bugs or other debris clogging its fins.

The condenser is a large radiatorlike device mounted immediately in front of the actual radiator. If the system fails to cool at all, the cause might be insufficient refrigerant, an electrical fault or system conflict. Newer A/C systems integrate operation with engine management strategy, adding an additional twist to diagnostics.

What is done during an A/C service? This varies widely depending on the shop, and includes some or all of the following: an outlet temperature test; a check of system pressures; belt and hose inspection; a leak check; and a condenser cleaning. Some shops might include a fungicide/odor treatment of the evaporator housing.

Bottom line? If your system blows sufficiently cool air while driving and at stops, and someone with trained eyes and ears is under the hood at least occasionally to spot potential trouble, save your money and put it into a nice vacation. If performance drops noticeably at stops or frequent compressor cycling is noted, attention is needed to prevent wear and tear on the compressor or its clutch.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose. Write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190; or e-mail to under-the-hoodjuno.com.