Why is my car’s air-conditioning not working?

Car air-conditioning used to be considered a luxury but more people probably have this feature in their cars than their homes today, given it’s been standard equipment for just about all new vehicles for many years now.

Indeed, we’ve become so used to jumping into our car on a hot day and switching the air-con to full blast to pull down the temperature – and to getting rid of damp air in winter – that when our AC system inevitably stops blowing cool, dry air anymore it’s a real let-down.

An air-conditioning system, in the most basic terms, uses a pump to feed refrigerant gas under pressure through a condenser, which typically looks like a small radiator that usually sits at the front of the car.

This gas is cooled in the condenser and then turns into a liquid. It travels through a drier then an expansion valve before entering the evaporator inside the car as a cold liquid.

The evaporator has a fan blowing air (coming from either inside or outside the car) over it. The air is cooled as it flows around the fins and tubes of the evaporator and then this cooled air is channelled through vents inside the car.

The liquid then starts to warm and turns into gas again as it repeats its path through the air-conditioning pump.

So what can you do if the AC stops blowing cold?

The first things to check are blockages – either at the condenser at the front of the car, particularly if you have a 4WD that had been driving in mud and debris has stuck to the condenser. You need to hose out the mud or clear away whatever other debris is blocking the fins of the condenser.

A malfunctioning module or a sensor may cause the fan to turn too slowly to cool the condenser. The same applies to a fan-clutch, which can operate at many rates according to needs.

The other potential issue is the pollen filter (also known as a cabin filter), which filters outside air as it comes into the cabin. Often, this filter is forgotten during servicing and can eventually become blocked with dust and therefore restrict air flow, potentially making the air not seem very cold.

Check in your owners’ manual or online where the filter is located (often it is a little hard to get to, under a cover under the bonnet or under the dash) and check it for heavy dust and leaves in the filter element. If the filter appears dirty, replace it with a new one.

Another potential trouble area is in a climate-control air-con system – that is, one that allows you to set the interior temperature – where the electrical components can become faulty.

In many cases, that can mean either the AC interior temperature sensor or solar sensor is not working properly.

Another problem (although this tends to affect older cars) is that the temperature blend motor has failed. The temperature blend motor controls a ducting flap under the dash via electrical input from temperature sensors to control the flow of air between hot from the heater or cold from the evaporator.

While these parts are usually not expensive, the labour time involved to get to them often is, as it usually means removing much of the dashboard. The same applies if it’s a leaking evaporator – it usually lives behind the dash and requires a lot of parts removal to get at what is usually an inexpensive part.

Another potential air-con failure point is the air-conditioning compressor, or its drive belt. The air-con compressor can simply stop functioning due to bearing failure or a seal failure.

Depending on the car involved, it is not always a cheap repair.

The most likely and common cause of an air-con system not blowing cold is low refrigerant gas in the system.

Although the pressurised air-conditioning system can keep its refrigerant level for 10 years or more, over time low-level leakage is inevitable.

Otherwise, component failure or a ruptured seal can cause a catastrophic leak.

Either way, the system needs to be recharged and checked for leaks by a professional.

Working with a car’s air-con pressure circuit is a bit like working on 240-volt appliances – it’s a no-no unless you are licensed to do so. This is one of those jobs where you need to take your vehicle to an auto electrician or an auto air-conditioning specialist for a repair.

Beyond some basic checks we’ve mentioned, there isn’t much else you can do to diagnose car air-conditioning issues without specialised training and specialised equipment.

But even so, the air-conditioning system is not an overly complex one in a modern vehicle.