Why should you run your air con in winter?

Yes, you should run your car air conditioning in winter, even though it feels odd with the heater on. Using it for a few minutes every week or two keeps the refrigerant oil moving around the system, which helps the seals and the compressor stay in good order over the months you would not normally touch it.

There is a second, more practical reason. On most cars the air con also dries the air going into the cabin, so pressing the button is often the quickest way to clear a misted windscreen. Many cars switch the compressor on automatically when you select the demist setting, so you may already be doing it without noticing.

Does air con work when the heating is on?

On most cars it does. The heater warms the air using heat from the engine coolant, while the air con compressor cools and dries the air before it reaches the heater. Used together, you get warm, dry air, which is exactly what you want on a damp morning.

Some cars with climate control manage this for you, and the A/C button may light up even when the system is set to a warm temperature. Others leave it entirely to you. Your handbook will say how your car behaves, because it varies between makes and models.

How often should you run the air con in winter?

There is no single rule for every vehicle, but a sensible habit is to run it for several minutes at least every week or two. Pick a cold setting, a moderate fan speed and let the engine reach normal temperature first. Check your handbook if you want the manufacturer view.

If you mostly make short trips, the system gets less chance to circulate its oil, so a deliberate run now and then matters a little more.

What can running it tell you about a problem?

A system that is used rarely can hide faults until the first hot day. Running it in winter gives you an early warning while there is time to do something about it. Things worth noticing:

• Air that never feels any cooler than the outside temperature
• A smell like damp socks or a musty cloth when the fan starts
• Unusual noises from under the bonnet when the compressor switches on
• A compressor that cuts in and out repeatedly
• Water pooling inside the car or misting that returns quickly

Why does my air con smell when I switch it on?

A musty smell usually points to moisture and growth in the evaporator box or a tired cabin filter. Regular use helps dry the system out, and a fresh filter is a sensible first step. Persistent smells can need the system inspected and cleaned.

A sweet, syrupy smell is different. That can suggest coolant escaping from the heater matrix, and you should not ignore it, particularly if the windows film over with a greasy mist.

What does a garage do and when should you stop driving?

A garage checks the system by looking at the compressor, belt, pipes and connections, testing how the air con performs and, where the equipment allows, checking whether refrigerant has been lost. Whether a recharge is enough or a part is needed depends on what the inspection finds, so we would not promise a fix before looking.

Air con faults rarely make a car unsafe by themselves. The exception is a squealing or smoking belt, or a temperature or battery warning light, because the compressor often shares a belt with other parts. Stop driving if any of those appear and have the car looked at.