The tyre pressure light is on and the tyres look fine

What the light actually monitors

Most systems use a sensor inside each wheel that reports pressure directly. Some infer it from wheel speed instead, which is cheaper and less precise.

Either way the light means the system is unhappy, not necessarily that a tyre is flat.

Check the pressures first

Cold, with a gauge you trust, against the figures on the door plate. A tyre can be twenty per cent down and look completely normal to the eye.

If one is low, you have a slow puncture rather than a sensor fault, and that is a £35 conversation rather than a £25 one.

A tyre visibly looking flat means it is very flat indeed. The eye is a poor gauge.

When it is the sensor

Sensor batteries last roughly five to ten years and are not replaceable separately. So on a car of that age, a light that will not clear after the pressures are correct is usually a dying sensor.

They also get damaged during careless fitting, which is an argument for whoever fits your tyres knowing what they are doing.

Cold mornings set it off

Pressure falls with temperature. A first cold snap triggers a wave of TPMS lights on cars that were perfectly fine a week earlier.

Reinflate to the plate figure and most of them clear. It is the single most common cause we see and it costs nothing to rule out.

What it costs

Reading and resetting the sensors is £25, done on the same visit as anything else. A replacement sensor is more, and we will tell you which you need before doing either.

Ignoring it is not really an option: a system that has been disabled or is faulty is an MOT failure on vehicles first registered after 2012.

Why it happens every November

Air contracts as it cools. A tyre set correctly at twenty degrees in August is meaningfully under-inflated at two degrees in November, without losing a molecule of air.

That is why the first real cold snap produces a wave of these calls, on vehicles that were fine a fortnight earlier. It is not a fault and it is not a puncture — it is physics, and the fix is a pump.

Check them cold, before driving. Ten minutes on a motorway warms a tyre enough to disguise a pressure that was low when you set off.

The first frost of the year is the busiest TPMS week we have, every single year.

What happens if you leave it

The immediate risk is that a genuine slow puncture is dismissed as a sensor fault and driven on. A tyre run at low pressure overheats, damages itself internally, and fails without much warning.

The secondary one is legal. On a vehicle first registered from 2012, a TPMS system that is faulty or has been disabled is an MOT failure in its own right.

Neither is an argument for panic. Both are an argument for checking the pressures properly rather than assuming the light is lying.