the TPMS light: what it means and why it comes on in the cold

the tire pressure monitoring light means at least one tire is low enough to matter. it is not a suggestion.

the two kinds of system

direct TPMS uses a battery-powered sensor inside each wheel that reports actual pressure. most vehicles since around 2008. accurate, and the sensors eventually die, usually between seven and ten years, because the battery is sealed inside and cannot be replaced.

indirect TPMS has no sensors. it watches wheel speeds through the ABS and infers that a tire is low because it is turning slightly faster. cheaper, no sensors to replace, and it has to be reset manually after inflating.

why it comes on every November

air contracts as it cools. pressure drops roughly one psi for every six degrees Celsius the temperature falls.

a tire set correctly on a warm October afternoon can be four or five psi low on the first hard frost, which is enough to trip the warning. the tire is not leaking. it is cold.

set them again once winter has actually arrived and check monthly through it.

when the light is telling you something worse

flashing for a minute or so at startup, then staying on does not mean a low tire. it means the system itself has a fault, usually a dead sensor. common on vehicles around the eight to ten year mark, and it means the system is not watching your tires at all.

on after inflating, on an indirect system, usually means nobody reset it.

coming back repeatedly on one wheel is a slow leak: a nail, a corroded rim, or a leaking valve stem. worth finding rather than topping up every week.

why it matters beyond the light

an under-inflated tire wears its shoulders, runs hot, uses more fuel and handles worse. it is also how a tire fails at speed, because heat is what destroys them.

the correct pressure is on the sticker in the driver's door jamb, not the number moulded into the tire sidewall. that one is a maximum, not a target, and people get this wrong constantly.