When your car’s air conditioning stops blowing cold, it can range from a simple refrigerant top-up to a compressor replacement. Before spending money at a workshop, understanding how the system works and performing some basic checks can help you identify the problem and decide whether it is a DIY fix or needs professional attention.
How Automotive AC Works
The AC system circulates refrigerant (R-134a on older vehicles, R-1234yf on most vehicles from 2017 onwards) through a closed loop. The compressor pressurises the refrigerant, the condenser (in front of the radiator) removes heat from it, the expansion valve or orifice tube drops the pressure, and the evaporator (inside the dashboard) absorbs heat from the cabin air as the cold refrigerant passes through it. Any leak or component failure in this loop stops the system from cooling.
AC Blows Warm Air
The most common cause is low refrigerant from a slow leak. AC systems lose small amounts of refrigerant over time through hose connections and O-ring seals. Common leak points include the compressor shaft seal, condenser (damaged by road debris), evaporator, and the service port valve cores. A professional AC service includes a leak test, evacuation, and recharge to the correct weight. Your workshop manual lists the exact refrigerant type and charge weight for your vehicle.
Compressor Not Engaging
If you turn on the AC and do not hear the compressor clutch click on (or see the clutch pulley engage), the system may be too low on refrigerant (the low-pressure switch prevents the compressor from running to protect it), the compressor clutch relay or fuse may have failed, or the compressor itself may be faulty. Check the AC fuse and relay first (your workshop manual identifies their locations in the fuse box). If the fuse is good and the relay clicks, the low-pressure switch may be cutting out due to low charge.
AC Blows Cold Then Warm
An AC system that cools initially then warms up can indicate the evaporator is icing over (caused by a faulty evaporator temperature sensor or expansion valve), a compressor that is cycling on and off due to borderline refrigerant level, or a blend door actuator fault in the climate control system that is mixing hot air from the heater with the cold AC air.
Bad Smell from the Vents
A musty or mouldy smell when the AC first turns on is caused by bacteria and mould growing on the damp evaporator surface. This is common in humid climates. An evaporator cleaner spray (applied through the blower motor opening or the cabin air filter housing) kills the bacteria and eliminates the smell. Replacing the cabin air filter at the same time is recommended. Running the fan on high without AC for a few minutes before turning the engine off helps dry the evaporator and prevents mould growth.
DIY vs Professional
Checking fuses, relays, the cabin air filter, and performing an evaporator clean are all DIY tasks. Refrigerant handling requires proper equipment and is best left to a licensed technician (in Australia, handling automotive refrigerant requires an ARC licence). Your workshop manual provides the diagnostic procedures, refrigerant specifications, and component locations for your AC system. MechanicMate offers PDF workshop manuals for over 960 models at mechanicmate.net/shop.
