Tyre pressure monitoring became mandatory on new passenger cars in the EU some years ago, and the requirement did not specify how it should be done. The result is two technologies with the same dashboard symbol and very different behaviour.
Direct TPMS: a sensor in each wheel
A battery-powered sensor sits inside each tyre, usually built into the valve stem, and transmits actual pressure — and often temperature — to the car by radio.
What it gives you: a real pressure reading per wheel, usually displayed as a number, and a warning that appears quickly when pressure drops.
What it costs you:
- The sensor batteries are sealed and not replaceable. Life is typically five to ten years, after which the sensor is replaced entirely.
- Sensors must be re-registered to the car after replacement or when swapping between summer and winter wheel sets. Some cars learn automatically after a short drive; many need a diagnostic tool.
- Tyre fitters can damage them. The valve core and the seal are consumables and should be replaced at every tyre change.
If you run two sets of wheels, you either buy two sets of sensors or accept a warning light for half the year.
Indirect TPMS: no sensors at all
Indirect systems infer pressure from the ABS wheel speed sensors. An under-inflated tyre has a slightly smaller rolling radius, so it turns marginally faster than the others. The car watches for that difference.
What it gives you: no extra hardware, no batteries, nothing to break, and no problem swapping wheel sets.
What it costs you:
- No actual pressure numbers. Only a warning.
- It must be reset manually after you inflate the tyres, because it works from a learned baseline. This is the step most drivers miss, and the reason the light “comes back for no reason”.
- It is poor at detecting all four tyres losing pressure together, which is exactly what happens over a cold winter, because the comparison between wheels still looks normal.
- It is slower to react and less accurate at speed extremes.
If your car shows a warning symbol but never a pressure figure, you almost certainly have an indirect system, and there is nothing inside the tyre to fail.
Aftermarket TPMS kits
For older cars, retrofit kits use screw-on caps or internal sensors and a small display on the windscreen or in a 12V socket.
The screw-on cap versions are appealing because fitting takes a minute. Their weaknesses are real: they are exposed to weather and theft, they add mass at the valve, and the seal they rely on is the cap itself — a poorly made one can slowly leak the tyre it is supposed to monitor. Internal retrofit sensors avoid all that but require tyre removal to fit.
For a retrofit kit, look for a stated water ingress rating, a display that dims at night, and a solar or USB-powered display rather than one with a small internal battery.
What the warning light actually means
A steady light means low pressure. A light that flashes for about a minute at start-up and then stays on means a system fault — a dead sensor, an unregistered wheel, or a receiver problem — not a flat tyre. That distinction is in the handbook of almost every car and is worth knowing before paying someone to diagnose it.
The habit that makes all of it less important
Monitoring is a safety net, not a substitute. Check pressures cold, once a month, with a gauge you trust, at the figure on the door pillar sticker rather than the one on the tyre sidewall — the sidewall number is the tyre’s maximum, not your car’s target.
