Car air con not cold or blowing warm air: 8 causes and the fix

Your air con is not cooling properly if the air from the centre vents is hardly colder than the outside air after a few minutes of driving. On a hot day a car that has been standing in the sun takes a while to cool down. That is normal. If the air stays lukewarm, something is wrong.

Check the controls first. Is the air con switched on (the A/C button)? Is the temperature set to the coldest setting? Is an automatic climate control system set to an ECO mode? Turn the fan up a few settings. Use recirculation (the button with the curved arrow) for a short time to cool down faster. Do not leave it on for long, because the windows then mist up more quickly.

Is no cold air coming out at all, or no air from the vents? Then our article car air con not working fits your problem better. This article is about an air con that still does something, but not enough.

When the air con cools poorly tells you a lot about the cause. So pay attention to the moment it happens: when the car is stationary or while driving, straight away or only after a while, on both sides or on one side. The table is a first clue, not a diagnosis.

A first clue based on the symptom. The cause only becomes clear from a measurement.

Below are the causes in the order we work through them in our workshop. We start with what happens most often and is easiest to measure. A fault can also have two causes at the same time.

1. Too little refrigerant due to slow loss
Refrigerant is the gas that carries heat out of the car in a closed circuit. Even a healthy system slowly loses refrigerant through hoses and seals. According to MAHLE that can be up to 10% a year. According to the ANWB (the Dutch motoring organisation), 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. You can read more on our page about an air con regas.

2. A leak
Does the air con cool poorly again within a few months of a refill? Then it is no longer normal loss. Leaks are often found at O-rings, hoses, the condenser and the service valves. We then pressurise the system with nitrogen. According to HELLA, a nitrogen test mainly finds larger leaks in places that are easy to reach. That is why we also use UV dye where needed, which glows under a UV lamp. If a leak is hard to find, we first discuss the next step with you. Read more about finding an air con leak.

3. A dirty or damaged condenser
The condenser sits at the front, behind the grille. It releases the heat from the refrigerant into the outside air. Leaves, insects, dirt and stone chips make that harder. The pressure then rises and the air con cools less, especially when the car is stationary and in traffic. If the condenser leaks or is badly damaged, it needs replacing. Read more about replacing the air con condenser.

4. A radiator fan that does not switch on
While you drive, the airflow passes through the condenser. When the car is stationary, the electric radiator fan has to do that work. If the fan does not run or runs too slowly, the air con cools poorly in traffic and well on the motorway. Causes include a faulty fan motor, a relay, a fuse or the fan control.

5. A worn compressor or clutch
The compressor (sometimes called the air con pump) pumps the refrigerant round the system. A worn compressor builds up too little pressure. The air con still cools, but weakly. A slipping magnetic clutch gives the same symptom. If the compressor does not engage at all, read our article air con compressor not engaging.

6. A faulty pressure sensor
The pressure sensor monitors the pressure in the system. If the pressure is too low or too high, the control unit switches the compressor off to protect it. If the sensor sends a wrong value, the compressor switches off even though nothing is wrong. HELLA names a faulty pressure sensor as a common cause of air con failure on certain Mercedes models. We compare the sensor value with the pressure measured by the service machine.

7. A flap or flap motor behind the dashboard
Behind the dashboard a flap blends cold air from the evaporator with warm air from the heater. A small electric motor, the flap motor (actuator), moves that flap into the right position. If the flap sticks, warm air is mixed in, even when the air con itself is cooling well. With dual-zone climate control you often notice this as a difference between left and right. Sometimes you hear ticking behind the dashboard.

8. A frozen evaporator or a clogged cabin filter
The evaporator sits behind the dashboard and takes the heat out of the air inside the car. If the temperature sensor at the evaporator does not work properly, or if there is too little refrigerant, ice can form on the evaporator. The airflow then gets weaker and weaker after a while of driving. A clogged cabin filter (pollen filter) also lets too little air through. On our page about cabin filter replacement you can read when a new one is needed.

With four easy checks you can narrow down the cause. You do not need any tools. Write down what you notice, so you can explain it clearly on the phone.

1. Compare standing still and driving. Let the air con run for a few minutes at idle and feel the air. Then do the same while driving. A big difference points to the condenser or the radiator fan.
2. Check whether the radiator fan runs. Switch the air con on while the car is stationary with the handbrake on. On many cars the fan at the front starts after a short time. Look through the grille and keep your hands, hair and clothing well away. An electric fan can start unexpectedly.
3. Look at the condenser. Are there lots of leaves, dirt or insects in front of the fins? Then have them cleaned. Do not use a pressure washer at close range, because the fins bend easily.
4. Compare left and right. Set both zones to the coldest setting. If one side stays lukewarm, the cause is probably a flap or flap motor and not the refrigerant.

A refill only helps if there is too little refrigerant and the system is sealed. With a leak the air con is empty again after a short time. The European directive on car air con systems (2006/40/EC, Article 6(3)) also says that a garage must not refill a system that has lost an abnormal amount of refrigerant until the repair has been completed. That is why we measure first and refill afterwards.

Topping up the air con yourself with a DIY can is not allowed in the Netherlands. Under the European F-gas Regulation (EU) 2024/573, only someone with a certificate or training attestation may work on a car’s air con. Non-refillable cans of refrigerant are banned in the EU. The ANWB also says that refilling the air con yourself is not permitted. Read more in can you recharge your car air con yourself? A can fills by guesswork. Too much refrigerant makes the pressure rise and the air con then cools worse, not better. A leak sealant can contaminate the system and the garage’s equipment.

During an air con service the service machine recovers the old refrigerant and weighs it. That shows how much was left compared with the factory amount on the sticker under the bonnet. We then put the system under vacuum. That removes moisture and shows whether the system holds pressure. Only then do we refill with the amount the manufacturer specifies.

A technician measures the pressure on the high and the low side, the vent temperature and how the fans and flaps work. Together these measurements show where the problem is. Our technicians hold an F-gas certificate for R134a and R1234yf. This is how we work:

1. You call with your registration number. Tell us when the air con cools poorly and whether it has been refilled before. Then we know whether to start with a leak test. We tell you in advance what the diagnosis costs.
2. We measure the vent temperature and the pressures. We measure the temperature at the centre vent and connect the service machine. Low pressure on both sides points to too little refrigerant. High pressure on the high side points to a condenser or fan that cannot get rid of the heat.
3. We read the fault codes. With diagnostic equipment we read the fault codes and the values of the sensors and flap motors.
4. Quote and your approval. We tell you what we have found and you receive a quote in advance. We only carry out repairs once you have agreed.

Would you like to know what an air con service includes and how to book one? Have a look at our page about car air conditioning in Rotterdam.

The cost depends on the cause. If the only problem is too little refrigerant, an air con service is enough. A new compressor or evaporator costs a lot more. That is why we start by measuring. If a part needs repairing or replacing, read more about air con repair.

Amounts marked “according to the ANWB” are market indications from the ANWB and not a quote from us. Our prices include VAT.

While you drive, the airflow cools the condenser. When the car is stationary, the electric radiator fan has to do that. If the air con cools poorly in traffic and well on the motorway, the condenser is often dirty or the fan does not switch on. Too little refrigerant can also play a part.

Then the cause is probably somewhere else, or the system is leaking. Think of a dirty condenser, a fan that does not run, a flap behind the dashboard or a weak compressor. Too much refrigerant after a refill without weighing also gives poor cooling. Measuring the pressures shows what it is.

You mainly notice it because the air gets less cold. Only a measurement gives certainty. During an air con service the service machine recovers the refrigerant and weighs it. Then you can see how much was left compared with the factory amount.

Yes, for the air con itself. For the engine it is usually not a problem. The refrigerant carries the oil that lubricates the compressor. If there is far too little, the compressor gets less lubrication. According to the ANWB (air con maintenance page, in Dutch), an air con that runs dry will break down. So do not wait too long.

Yes. In very hot weather the air con has to remove more heat and it takes longer before the car is cool. Open the windows briefly when you get in and use recirculation for a short time. If the air is still lukewarm after a few minutes, something else is going on.

Yes. Electric cars and hybrids also use refrigerant. In many of these cars the air con also cools the battery. If it works poorly, the car may limit fast charging or power. Our technicians hold a GMTO certificate, the Dutch qualification for working on high-voltage systems.

No. The APK (MOT), the Dutch periodic roadworthiness test, does not check whether the air con cools. The European rules for periodic testing only mention demisting the windows, and not as an essential item. An air con that cools poorly is therefore no reason for the car to fail. A working air con is useful for clear windows in winter, though.

• ANWB: De airco van mijn auto werkt niet goed: wat nu? (in Dutch)
• ANWB: Airco onderhoud: dit moet je weten (in Dutch)
• MAHLE: AC-check flyer: refrigerant loss of up to 10% a year (in Dutch)
• HELLA: Car air-conditioning leak detection
• EUR-Lex: Regulation (EU) 2024/573 on fluorinated greenhouse gases
• HELLA: Mercedes W168/W210: Air conditioning system failure