Why a Car's Air Conditioning Runs but Does Not Cool

An air conditioning system that runs and produces no cold air is one of those faults where the compressor running is easy to see and the actual problem is harder to reach. A system that does not cool at all has a small number of causes, and identifying which one applies from the pattern of when it fails saves a great deal of diagnostic time.

This guide covers how the system works and how to find the fault.

How the System Works

The refrigerant circulates in a closed loop. A compressor driven by the engine raises the pressure of the refrigerant, which makes it hot enough to give up heat to the outside air. That heat goes out through the condenser, which sits in front of the radiator. The refrigerant then passes through an expansion valve or orifice tube, where the pressure drops and it becomes a cold low-pressure vapour. That cold vapour absorbs heat from the air inside the cabin as it passes over the evaporator, which is what produces the cold air. The refrigerant then returns to the compressor as a low-pressure vapour, and the cycle repeats.

Every one of those steps has to be happening for the air to be cold, and each has its own failure mode with a different set of symptoms.

The Most Common Causes

The System Is Simply Low on Refrigerant

By far the most common cause, and the one people expect. A leak, an open condenser valve, or a failed seal reduces the charge, and a system with insufficient refrigerant cannot produce the pressure differential needed to cool properly. The typical behaviour is that it cools for a while and then the cold air stops, particularly after the system has been running for some time, because the remaining charge circulates for a while before it is insufficient.

Finding the leak is the actual work. Adding refrigerant without finding it means the fault returns within weeks or months, and repeated charging progressively damages the system. A UV dye added to the charge will show the location under a UV light, and dye that has been in the system long enough accumulates at the leak point even after the pressure has been lost. The costs escalate quickly here: refrigerant alone is cheap, and a compressor that has been run with no charge can be destroyed in minutes, so a system that has run flat is a compressor at risk.

A Thermostat or a Temperature Sensor

On most systems there is no separate thermostat, but the evaporator temperature sensor controls the compressor cycling. A sensor that reports falsely warm causes the compressor to cut out, and the symptom is an air conditioning system that runs for a short period and then stops, leaving the air warm. A sensor reporting falsely cold causes the compressor to run continuously, which is the opposite symptom and is also easy to identify. A compressor that never cuts out at all is a common fault on older vehicles.

A Failed Compressor

A compressor that has failed mechanically, commonly because it was run without refrigerant, presents as an air conditioning system that does not engage at all. The mechanical noise is usually different, and where the system engages and the vents move air that is not cold, the compressor is not producing pressure. On many vehicles the compressor clutch or the electromagnetic coil is what fails rather than the compressor itself, which is a much less expensive repair.

A Failed Condenser Fan

The condenser has to reject heat, and it needs airflow to do it. On many vehicles the condenser fan runs when the air conditioning is engaged, and on some it shares a fan with the radiator. Where the condenser fan is not running, the system cannot reject heat, and the symptom is air conditioning that cools briefly and then goes warm again, or that is worse in slow traffic than on a motorway, because there is no airflow. This is a very common and inexpensive fault and it is frequently overlooked because people expect the compressor to be the problem.

An Electrical or a Control Fault

The compressor clutch, the pressure switch, the pressure sensor, the control module, and the fuse all sit in the path. A fuse that has failed, a relay, or a pressure switch that has failed to switch will prevent the compressor engaging. A vehicle that has no air conditioning at all, from cold, points at this rather than at a refrigerant level, and it is worth checking the simplest items before assuming anything more serious.

A Stuck Blend Door or a Failed Actuator

On a vehicle with a blend door or with individual zone actuators, a door that has stuck in the recirculation or the hot position will blow air that never passes across the cold evaporator, even though the system is working perfectly. The symptom is air that changes temperature with the mode selection but never gets cold in any mode. A clicking sound when changing mode indicates a stripped actuator gear.

A Blocked or Iced Evaporator

The evaporator can freeze if the expansion valve has failed and passes too much refrigerant, or if the airflow through the evaporator is restricted. Once it freezes, the ice blocks the airflow and the air goes warm, and it may clear and refreeze repeatedly. The sign is air that is cold for a few minutes then goes warm, and often a hiss of air rather than a cold blast. A failed blower motor or a clogged cabin filter is what usually causes the airflow restriction.

Diagnosing It

Establish when the system loses cooling. Cold for a few minutes then warm, immediately warm, warm only in traffic, warm at all speeds, and the air temperature depending on the mode, each point to a different cause. The first question is whether the compressor engages, and that is visible from the engine bay: on a manual or a switchable compressor, the clutch will visibly click. On an automatic compressor it is not visible and the observation has to be made differently.

Then check whether the condenser fan runs when the air conditioning is engaged, which is a two-second check and identifies one of the most common causes. Check the temperature actually delivered at the vent with a simple thermometer rather than judging by hand. Check whether the air coming out is cold at all, since cold-then-warm and never-cold are different faults.

Check the fuel trims, because a system that has been out of refrigerant for a long time may have a failed compressor that has contaminated the system with metal debris, and that debris will damage a replacement compressor if the rest is not properly cleaned. This is why the repair quote for a compressor is higher than the part price.

What to Do

If the system is not cooling at all, have the charge checked and the leak found. If it is a fuse or a relay, that is trivial. If the compressor clutch or coil has failed, that is a moderate repair. If the compressor itself has failed, that is an expensive one, and on an older vehicle it is worth asking whether a system that still cools for a while in cool weather is worth a full repair or whether the vehicle's remaining value justifies it.

Do not add refrigerant speculatively. An incorrect charge is as bad as no charge, and a system that has been charged without the leak being found will fail again. The refrigerant specification matters and is not universal, and mixing types causes problems that are not immediately visible. The specification reasoning is in how to choose the right coolant, which covers the same principle in the cooling system, and the general diagnostics are in our vehicle maintenance guides.

If this guide was helpful, you may also want to read how to stop your car cabin smelling, because a failed evaporator drain causes the damp that produces a musty smell, and the two faults frequently occur together.