How does car air conditioning work? Every part explained

The air con works on a basic principle of physics: a liquid that evaporates absorbs heat. In the air con this happens with refrigerant, usually R134a or R1234yf. The refrigerant goes round and round and keeps changing between gas and liquid. One cycle goes like this:

1. Compressor. Squeezes the refrigerant gas. Pressure and temperature go up.
2. Condenser. Sits in front of the engine radiator. The hot gas releases heat into the outside air and turns into a liquid under high pressure.
3. Receiver drier. Removes moisture and dirt from the refrigerant. Systems with an orifice tube have an accumulator on the low-pressure side instead.
4. Expansion valve or orifice tube. The liquid flows through a narrow opening. The pressure drops and the refrigerant cools down sharply.
5. Evaporator. Sits behind the dashboard. The refrigerant evaporates and absorbs heat from the air that the blower fan pushes past it. That air comes out of the vents cool and dry.
6. Back to the compressor. As a gas under low pressure. Then the cycle starts again.

So the air con has a high-pressure side, from the compressor to the expansion valve, and a low-pressure side, from the expansion valve back to the compressor. The refrigerant contains oil. According to the ANWB (the Dutch motoring organisation), the circulation of the refrigerant lubricates the compressor. Which refrigerant your car uses is explained in R134a or R1234yf: which refrigerant does your car use. How much belongs in the system is explained in how much refrigerant is in your car’s air con.

For each part: where it sits, what it does and which fault often comes with a defect.

How the parts work according to HELLA and the ANWB. The faults are common signs. One fault can have several causes, and a measurement gives certainty.

The compressor pumps the refrigerant round and builds up the pressure. In a car with a petrol or diesel engine, the engine drives it through the auxiliary belt. A clutch switches it on and off when cooling is needed. In electric cars and some hybrids the compressor has its own high-voltage electric motor. HELLA writes that this type of compressor works independently of the engine. That means the air con can also cool while the engine is off, for example to pre-cool the car.

According to the ANWB, a compressor lasts at least 6 years or 120,000 km with normal use and maintenance. The compressor needs oil, and that oil travels with the refrigerant. An air con that runs for a long time with too little refrigerant therefore puts extra strain on the compressor.

Do you hear rattling or humming? Read car air con noises. Does the compressor not engage? Read air con compressor not engaging. Thinking of a replacement? We first measure whether the compressor really is the problem. See air con compressor replacement in Rotterdam.

The condenser releases heat into the outside air. It sits at the front, often in front of the engine radiator, where it catches stone chips, dirt and road salt. According to the ANWB, a condenser lasts at least 5 years or 100,000 km with normal use and maintenance. If a condenser leaks, we replace it after a leak test. See air con condenser replacement.

The evaporator does the opposite and absorbs heat from the air inside the car. It gets cold, and moisture from the air settles on it. That water drains away under the car through a small drain hose. A small puddle under the car after a drive with the air con on is therefore normal. If the water ends up inside the car, read car air con leaking water. MAHLE calls the condensation on the evaporator a breeding ground for bacteria and mould. You notice that as a musty smell. See why your car air con smells musty (in Dutch) and car air con cleaning.

Replacing a leaking evaporator is a big job, because the dashboard often has to come out. Read more on air con evaporator replacement and in how much does a car air con repair cost.

The receiver drier is a small canister with a filling that absorbs moisture. Moisture does not belong in the air con. Together with the oil it attacks the parts, and it can freeze in the expansion valve. MAHLE writes that the drier can only absorb a limited amount of moisture, and advises replacing it as a precaution on older cars or cars with a high mileage. After a repair in which the system has been opened, we check whether the drier needs replacing as well.

The expansion valve controls how much refrigerant flows into the evaporator. Other cars have an orifice tube: a small tube with a fixed opening. Those systems have an accumulator instead of a receiver drier.

The pipes and hoses connect everything. According to MAHLE, an air con system can lose up to 10% of its refrigerant a year. A real leak is often found at a joint, a hose that chafes or a corroded pipe. How we find a leak is explained on our page about air con leak detection in Rotterdam. For repairs, read leaking air con pipe or hose: repair or replace.

The air con dries the air, because moisture from the air condenses on the cold evaporator. That makes the air con a quick way to clear misted-up windows. The ANWB describes the air con as a handy tool against misted-up windows in colder periods.

In a petrol or diesel car the heat from the heater comes from the engine coolant, not from the air con. In winter the air con dries the air first, and then the heater warms that dry air up again. If the heater blows cold air, the air con is usually not the cause. See car heater blowing cold air.

So use the air con all year round. The ANWB writes that it is important to use the air con throughout the year, including in winter, and to have it checked every year. According to the ANWB, refrigerant slowly escapes from an air con that is not used for a long time. Also read misted-up windows? Switch on the air con, in winter too (in Dutch).

In an electric car the cycle works in the same way, but with three differences:

Nissens lists R134a, R1234yf and R744 (CO2) as refrigerants used in car heat pumps. R744 is used in, among others, Volkswagen ID models with a heat pump and the Audi Q4 e-tron with a heat pump. We work with R134a and R1234yf. Does your car use R744? Then call us first with your registration number.

Our technicians hold a GMTO certificate, the Dutch qualification for working on high-voltage systems in hybrid and electric cars. Read more about the air con in a hybrid or electric car (in Dutch), air con service for your electric car in Rotterdam and how much fuel or range the air con uses.

According to the ANWB, the refrigerant needs topping up on average once every 4 to 6 years. So have the air con checked every year. If it cools less well, an air con service is needed. During an air con service the technician recovers the refrigerant, puts the system under vacuum, checks that it is sealed and fills it with the specified amount. Topping up yourself with a DIY can is not allowed. Read can you top up your car air con yourself.

Everything about the service is on our page about car air conditioning in Rotterdam, and the schedule is in how often your car air con needs maintenance (in Dutch). Do you have a problem? Start here:

Would you like us to measure what is going on? We tell you in advance what the diagnosis costs, and we only carry out repairs once you have agreed. Read more about repairs on our page about car air con repair in Rotterdam.

No. Not in a petrol or diesel car, because the engine drives the compressor. In an electric car and some hybrids it does, because their electric compressor works independently of the engine. That is why you can pre-cool those cars.

No. The air con is the cooling system. Climate control is the control system that automatically adjusts the air con, the heater and the ventilation to keep the temperature you have chosen.

When the car is stationary, little air flows through the condenser, so the radiator fan has to do that work. If the fan is weak or the condenser is full of dirt, the air con cools less well in traffic. A shortage of refrigerant also often shows up in traffic first.

No. The APK (MOT), the Dutch periodic roadworthiness test, does not check the air con. An air con that does not work is therefore no reason for the car to fail.

According to the ANWB, the compressor lasts at least 6 years or 120,000 km with normal use and maintenance, and the condenser at least 5 years or 100,000 km. Regular use and a yearly check help the system reach that.

Yes. In a petrol or diesel car the engine has to drive the compressor, and in an electric car the compressor uses battery power. How much depends on the weather and how you use it. Read more in how much fuel or range the air con uses.

• HELLA: A/C compressors: how the air conditioning circuit works
• ANWB: Airco in de auto (in Dutch, consulted November 2026)
• ANWB: Airco en onderhoud (in Dutch, consulted November 2026)
• MAHLE Aftermarket: AC-check flyer: air con check and maintenance (in Dutch)
• Nissens: Poster: heat pump systems in electric vehicles (in Dutch)