Car aircon not cooling: the causes, in the order to check them
When a car aircon blows but the air is not cold, the cause is nearly always one of seven things, and three of them can be checked in a driveway in a few minutes: the cabin filter, the condenser behind the grille, and whether the compressor is engaging at all. The others, the refrigerant, the compressor itself, the blower and the wiring, need a shop with gauges.
Aircon repair is not a job this crew does. The crew does the cabin: vents, dashboard, carpets and the smell, as part of the interior detail. What each part of the system does, and what to ask a shop, is on the car aircon page. This page is the order to check things in before you spend anything.
Five minutes before anything else
- 1. The controls. A/C button lit, temperature at its coldest, recirculate on, fan on a middle speed. A car that has sat in the sun takes a few minutes to pull the heat out of the cabin, so drive with the windows down for the first minute before judging it.
- 2. The click. Engine idling, hood up, press the A/C button off and on. The compressor clutch should click and the engine note should change slightly. No click means the system is being told not to run: a fuse, a relay, a pressure switch, or refrigerant so low the cut-out has tripped. A click and warm air means the problem is further round the loop.
- 3. The condenser. Look through the grille at the thin radiator in front of the main one. Mud, leaves, a plastic bag, flattened fins. With the A/C on at idle, the fan behind it should be running.
- 4. The cabin filter. Behind the glovebox on most cars. Pull it. If it is grey and packed, the airflow is choked and that alone can make cold air feel warm at the vent.
- 5. The fuse. The A/C fuse and relay are in the fuse box, marked in the manual. A blown fuse is a minute to swap and a sign that something made it blow.
The causes, most likely first
- 1. Low refrigerant, from a slow leak
- What you notice. The cooling got weaker over weeks. Cold for the first minutes of a drive, then not. A hiss near the dashboard, or oily dust on a fitting under the hood.
- Who fixes it. A shop, with gauges. Refrigerant is not used up, so low means a leak, and a top-up without a leak test comes back warm in a month.
- 2. A blocked cabin filter
- What you notice. Weak airflow at every fan speed, a dusty smell, air that is cold at the vent but there is too little of it.
- Who fixes it. You. Pull it and replace it. It is the cheapest cause on the page and the one most often missed.
- 3. A dirty or blocked condenser, or its fan not running
- What you notice. Cold on the highway, warm in traffic and at red lights. Worse on hot afternoons. The classic one on a slow road.
- Who fixes it. You can look, and rinse the fins gently from the front with a hose and the engine off. Never a pressure washer; the fins fold flat. A fan that does not run with the A/C on is a shop job.
- 4. The compressor not engaging, or failing
- What you notice. No click when the A/C is switched on. Or a click followed by a squeal, or a grinding noise that comes and goes with the A/C.
- Who fixes it. A shop. The car’s computer also cuts the compressor when the pressure is wrong, so no click can be a refrigerant problem wearing a compressor disguise.
- 5. The blower motor or its resistor
- What you notice. Air only on the highest fan speed, or none at all, while the system itself may be perfectly cold with nothing to move the air.
- Who fixes it. A shop. The resistor is often reachable; the motor is behind the dashboard.
- 6. A stuck blend door
- What you notice. Warm air mixing in whatever the dial says, or cold on one side of the car and warm on the other.
- Who fixes it. A shop. A flap inside the dashboard that has stopped moving.
- 7. A fuse, a relay or a pressure switch
- What you notice. Everything dead at once, or the compressor never engaging with the refrigerant level fine.
- Who fixes it. The fuse is yours. The relay and the switch are a shop’s, and so is finding out why the fuse blew.
Why it cools on the highway and gives up in traffic
The condenser gives up its heat to air moving through it. At speed the road supplies that air. At a standstill the fan behind the grille has to supply all of it, and if the fan is weak, or the fins are packed with mud, the refrigerant arrives at the evaporator still warm. The vents go lukewarm in the queue and come back cold when the traffic moves. An hour on a slow road is the test that finds this, and it is the test most cars here get every day.
A system that is slightly low on refrigerant shows the same pattern, so the two get confused. The difference is that the condenser one gets worse on hot afternoons and better after a rinse of the fins, and the refrigerant one gets worse every week whatever the weather.
If the air is cold but it smells
That is a different problem and the right page is a different one. A musty smell for the first minute after the aircon starts is mould on the damp evaporator fins, and the fix is an evaporator clean. A smell that stays is in the carpet or the seats, usually from water that got in, and that is a cabin job. Which is which is on car smells and stains, and where damp hides after the rains is on mold in a car interior.
Glass that fogs when the aircon is on in the rain is also not a cooling fault. Which side the fog is on, and the fix for each, is on foggy windshield in the rain.
Do not buy the recharge can first
A can of refrigerant from the hardware store is the most common first move and the wrong one. If the system is low, it is low because of a leak, and the can goes out through the same hole. If the system is not low, the can overcharges it, and an overcharged aircon cools badly too. Either way the shop now has to recover whatever is in there before it can take a true reading.
A shop with gauges reads both sides of the loop, finds the leak with dye or a detector, repairs it, pulls a vacuum to take the moisture out, and then weighs in the amount of refrigerant the car was built for. The questions to ask before leaving the car with one are on the car aircon page.
Book the cabin clean, take the cold to a shop
If what you have is the smell, the dust in the vents or a cabin that has been damp all season, book the interior and exterior detail and say in the form that the smell comes on with the aircon. The crew works at your address with the doors open and brings its own power. If what you have is warm air, take the list above to a shop and start with the cabin filter and the condenser, because those two are free to check.
Sources
- Motorist (Thailand), car air conditioner not cold, causes and basic fixes before visiting a service centre: refrigerant, cabin filter, compressor, condenser fan, fuses, and the checks to run first. motorist.co.th
- Christian Brothers Automotive, my car’s A/C isn’t cold: leaks, the computer shutting the compressor off for an electrical fault, low refrigerant or overheating, the blend door, and the blower fan. cbac.com