How your car's air conditioning works

A car's air conditioning is a sealed loop. A compressor driven by the engine (or an electric compressor in many hybrids and electric vehicles) pressurizes refrigerant, a condenser in front of the radiator sheds heat, an expansion valve or orifice tube drops the pressure, and an evaporator behind the dashboard gets cold. A blower pushes cabin air across the evaporator. If any part of that loop leaks, fails or clogs, cooling suffers.

Common AC problems

Slowly getting weaker. Usually a leak. Refrigerant doesn't get used up; if it's low, it went somewhere.
Blowing warm right away. The compressor may not be engaging: a fuse, relay, pressure switch, clutch or the compressor itself.
Cold at highway speed, warm at idle. Often a condenser airflow problem: a failing cooling fan or a condenser clogged with debris.
Musty smell. Moisture and growth on the evaporator, or a dirty cabin air filter.
Water on the passenger floor. A clogged evaporator drain. Clear water dripping under the car on a hot day is usually normal condensation.

Why a recharge alone isn't a repair

Adding refrigerant to a leaking system can bring back cold air for a while, but the refrigerant will leak out again. A proper repair finds the leak, with dye, an electronic detector or pressure testing, fixes it, evacuates the system to remove air and moisture, and recharges it to the maker's specified amount. Federal rules govern how refrigerant is handled and recovered; see the EPA's motor vehicle air conditioning program. Ask the shop how they'll find the leak.

Refrigerant types

Many older vehicles use R-134a. Many newer ones use R-1234yf. They aren't interchangeable, and each needs its own service equipment. The label under the hood says which yours uses. Ask whether the shop is equipped for your vehicle's refrigerant.

How the heater works

Your heater uses heat from the engine. Hot coolant flows through a small radiator called the heater core behind the dashboard, and the blower pushes air across it. Electric vehicles use electric heaters or heat pumps instead.

Common heating problems

No heat or weak heat. Low coolant, a thermostat stuck open, a clogged heater core or air in the cooling system.
Heat on one side only. A blend door or its actuator may be stuck.
Clicking behind the dashboard. Often a failing blend-door actuator.
Sweet smell or foggy windshield. Possibly a leaking heater core. Don't ignore it; coolant is toxic and a leak can get worse.
Blower works only on high. Often a failed blower resistor or module.

Defrost and visibility

The defroster uses the air conditioning compressor to dry the air, even in winter, which is why a broken AC can mean a slow-clearing windshield. That makes AC repair a visibility issue as well as a comfort one.

Cabin air filters

Most newer vehicles have a cabin air filter that cleans air coming into the car. A clogged filter cuts airflow and can smell musty. Your owner's manual gives the replacement interval; dusty conditions may call for more frequent changes.

Questions to ask

How will you find the leak, and what did you find?
Which refrigerant does my vehicle use, and are you equipped for it?
Will you evacuate and recharge to the specified amount?
Is the coolant level and condition right for the heater to work?
When was the cabin air filter last changed?
What's the written warranty on the repair?