A couple of weeks back you told how to check your air conditioning system to be sure it was working properly. Mine unfortunately didn’t pass the temperature test you mentioned. What can I expect if I take it in for service? My repair shop is offering an “air conditioning service” special for $99. Will this correct the problem or could I be in for a larger bill?

-Molly Sola, Alameda, Calif.

Molly, you’re one of several people to ask this. Let’s start with a look at some of the reasons an A/C system might fail to function properly, then we can tackle the service options.

The most common condition and owner complaint is the system blows warm air. This is often the result of insufficient or no refrigerant – because of leakage or an electrical fault disallowing compressor operation. A system that blows barely cool air or intermittently cool air might be suffering from moderately insufficient refrigerant, condenser cooling fan problems or a mix-up of heated and chilled air in the air-distribution system. It’s also possible the refrigerant control device might be contaminated with debris or be faulty.

A worst-case scenario is compressor failure, which involves not only replacement of this expensive part, but careful flushing of debris from the system’s many passages.

Refrigerant leakage is common, as the tiny refrigerant molecules can sneak past worn or dry seals, and through rubber hoses, and the system is under continuous pressure, even when inactive. In severe cases, corrosion-caused perforations or damage to the aluminum condenser or evaporator can lead to rapid leakage and a hefty repair bill. Determining the cause of leakage and correcting it is essential to protect the environment from escaping refrigerant and to provide a lasting repair. R-134a, the more environmentally friendly refrigerant used since the early ’90s, has a greater tendency for leakage than the older and now very expensive R-12 refrigerant used in the past.

What’s involved in an A/C service? This is a real wild card. At the very least, this service should include a system performance (temperature and air distribution) check; visual inspection of hoses, fittings and the compressor drive belt; and an accurate estimate for additional work needed. Additional inspections might include refrigerant identification (important on older, often compromised R-12 systems), a refrigerant leak check and light diagnosis of possible electrical faults.

On R-134a vehicles, because of the modest price of refrigerant, it’s possible a shop might add a pound of refrigerant to top off a moderately low system with no obvious leaks. This is a poor practice, though, as it’s virtually impossible to determine system fullness, and an overcharge condition can cause problems. The right way to add refrigerant is to evacuate the system fully and recharge with the specified quantity. Leaking R-12 systems require repair and retrofitting to R-134a, because of the astronomical price of the older, no longer manufactured refrigerant.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif. E-mail him at [email protected]