The air conditioner is a group of several components that are responsible for removing heat from the interior of the vehicle, and introducing cool air into the vehicle. Specific parts of the A/C system perform this function by changing the refrigerant to liquid or gas as needed to cool the evaporator. Air is then passed across the cooled evaporator, and into the cabin for the occupant's comfort.

If your A/C is providing little cooling or none at all, it may be that the system is leaking A/C refrigerant. As the system becomes low on refrigerant, you may hear the A/C compressor cycling on and off rapidly when the A/C is turned on.

When the air conditioner fails, the most common symptom is warm air from the vents, but squeaking or grinding noises may be heard from the engine, even through the A/C is working normally. When the air conditioner is activated, the engine idle speed will not dip as it normally would.

In most situations, a vehicle with a faulty air conditioner is perfectly safe to drive. However, if grinding is heard from the compressor pulley, bearing failure may occur, causing the serpentine belt or drive belt to work its way off the pulley. If the belt breaks or falls off, other necessary belt driven components will stop working as well. If this occurs, the battery warning light will illuminate, and the engine will stall due to lack of electrical power.

Air conditioning diagnostics are so common that many service shops are specialized in heating and A/C repair for motor vehicles. The chances of having the air conditioning serviced during a vehicle's lifetime is very high, but not typically before the 100,000-mile mark has been reached. The most common failures are expansion valves, orifice tubes, and accumulators, since those components are more prone to clogging with contaminants.

Diagnosis of the air conditioning system requires advanced knowledge of air conditioning systems, and a good understanding of the relationship between temperature and pressure of a liquid and gas. A technician with proper training, and certification is required to repair an air conditioning system. Professional grade equipment is used to read high and low side pressure, and the technician must rely on hours of classroom and hands-on training to understand the readings, diagnose the faulty part, and recommend a repair.

Adding refrigerant to a poorly functioning AC system without first diagnosing the problem may result in overcharging. This has been known to damage the AC compressor, resulting in a very expensive (and avoidable) repair.

Replacement of most air conditioning components requires complete evacuation of the air conditioning system with a refrigerant recovery machine. Once components are re-installed, the technician will remove all air from the system in preparation for recharging the system with new refrigerant.

Many customers claim that their "A/C is not getting cold" when the actual issue is too little airflow from the vents. When you take your vehicle in for service, try to demonstrate the problem so the shop has a better understanding before they start the diagnosis.