Why does my tyre pressure warning light come on in cold weather?
Because air contracts as it cools, so the pressure inside a sealed tyre genuinely falls — no leak required. The rule of thumb is roughly 1 psi lost for every 10°F drop, or about 0.07 bar for every 5.5°C.
An overnight fall from 20°C to 0°C can therefore take a tyre from correctly inflated to below the warning threshold, which is why the light so often appears on the first genuinely cold morning of the year and on no particular day before that.
The system reporting it is TPMS, tyre pressure monitoring. There are two kinds. Direct systems use a pressure sensor inside each wheel and can usually tell you which tyre and what pressure. Indirect systems infer under-inflation from wheel speed sensors — an under-inflated tyre has a slightly smaller rolling radius and turns marginally faster — and can only tell you that something is low.
The correct response is to check all four with a gauge when the tyres are cold, meaning before driving or at least three hours after. Driving heats the air and raises the reading by several psi, so a pressure set at a warm tyre will be too low once it cools.
The correct pressures are on a placard in the driver's door jamb, inside the fuel flap, or in the manual — not the number moulded into the tyre sidewall, which is the maximum the tyre can take, not the recommended figure.
If the light returns within days after correct inflation, you have a slow puncture rather than a temperature effect.
A flashing rather than steady TPMS light usually indicates a system fault, often a sensor battery, not low pressure.