Why does my tyre keep losing pressure?

Every tyre loses a little air, about 1 psi a month. But if one tyre needs topping up every week or two, or the warning light keeps coming back, something is letting air out. It's usually slow and usually cheap to fix, but worth sorting: a slow leak can turn into a fast one.

One tyre, or all four?

If all four are a bit low after a cold snap, that's the temperature: air shrinks as it cools. Here's more on cold weather. If it's one tyre, it's a leak, and it's almost always one of these.

The usual causes

• A nail or screw in the tread. By far the most common. The object often half-seals the hole, so the air seeps out slowly over days. Leave it in until a fitter can look at it.
• The valve. Rubber valve stems perish and crack with age, and the small valve core inside can work loose or leak. A missing valve cap lets dirt in. A new valve is cheap, and fitters normally fit one with every new tyre.
• Where the tyre meets the wheel. On alloy wheels, corrosion builds up on the rim, especially after a winter of road salt, and the tyre can't seal against it. The fix is to take the tyre off, clean the rim and refit it.
• A damaged wheel. A hard pothole hit can bend or crack the edge of the rim. Sometimes you can see it; sometimes it needs a wheel balancer to show it up.
• A tyre pressure sensor. On cars with sensors in the wheels, the sensor doubles as the valve, and its seals can leak with age.
• Damage to the tyre itself. A cut or a bulge in the sidewall. That tyre needs replacing, not topping up.

Finding the leak yourself

• Pump the tyre up to its normal pressure first, so the leak is easier to find.
• Look over the tread for anything stuck in it, rolling the car a little to see all the way round.
• Spray soapy water on the tread, round the edge of the rim and on the valve, with the cap off. A stream of tiny bubbles shows you where the air is coming out.

What to do about it

Don't keep topping it up for weeks. Every time it runs low you're driving on a soft tyre, which wears it and can damage it inside. Take it to a tyre fitter. Most slow punctures can be properly repaired if the hole is in the central three-quarters of the tread and no bigger than 6mm, which is the British Standard for tyre repairs, BS AU 159. A proper repair is done from inside, with the tyre off the wheel.

Once it's fixed, set all four tyres to your car's figures and, if your car needs it, reset the warning light.