Tyre pressure sensors: a helper or a new expense?

Why does a car have pressure sensors?

When I examined run-flat tyres, I firmly advised against fitting them to cars without a tyre pressure monitoring system, which at the time was mostly fitted to premium models. Since then lawmakers have changed everything.

Under Regulation (EC) No. 661/2009, the system has been mandatory for new car models since November 2012 and for all new passenger cars registered in the EU since November 2014; since 2022 this requirement has been taken over and tightened by Regulation (EU) 2019/2144. In the United States it has been required on all new passenger cars and light trucks since September 2007. So many of the cars brought into Ukraine have it.

And what must it actually be able to do? Under the European requirements (UN Regulation No. 141) it must warn the driver within 10 minutes of driving when the pressure in at least one tyre has fallen by 20% from the working pressure or dropped to 1.5 bar; the American standard lets it wait until the pressure has fallen by a quarter. In other words, a tyre pressure monitoring system is an alarm for trouble, not a pressure gauge. It is like the low-fuel warning light: it comes on when the tank is nearly empty, not when it is convenient to fill up.

Direct and indirect: what is the difference?

A direct system is a battery-powered sensor in each wheel, usually on the valve stem on the inside of the wheel. It measures pressure and temperature and sends them by radio.

An indirect system has no sensors of its own in the wheels: it uses the wheel-speed sensors of the ABS. An under-inflated tyre has a slightly smaller rolling radius and makes more turns than its neighbours, and that is what the electronics notices. It is cheap and there is nothing to break, but it is less sensitive: such a system passes the certification test, yet on the road it is slightly inferior to the direct one.

And in practice? A 2018 survey of more than a thousand cars on European roads showed that at least one tyre under-inflated by 0.3 bar or more was found on about a third of the cars with an indirect system, on slightly fewer of those with a direct one, and on over 40% of older cars without a system. Pressure sensors help noticeably, but they do not make the gauge in the glovebox redundant.

Why does the light come on with the first frosts?

Have you never noticed that the pressure light most often “breaks” on the first frosty morning after the tyre changeover? And the tyre fitter is right there: “the sensor’s dead, it needs replacing”, or even “top up with nitrogen, it doesn’t depend on temperature”. In fact the sensor is most likely fine, and nitrogen, like any gas, obeys Gay-Lussac’s law, which I covered when going through the myths about gas in tyres.

Let’s do the sum. A tyre is inflated to 2.3 bar in a warm bay at +20 °C. At night it is -10 °C and the pressure falls to about 1.95 bar, almost 15%. Another ten degrees of frost or a small leak, and the light comes on quite legitimately.

And here I owe you an explanation. In the article on tyre pressure I thought it acceptable to “let down” the tyres by 10-15% in winter, and on low pressure I weighed up its pros and cons. On a car with sensors that advice no longer works: deliberate under-inflation plus frost means losing a quarter, even a third, of the pressure, and the light will be on for good reason. And an indirect system reset at low pressure will take it for the norm and warn you of a real puncture later.

So inflate the tyres exactly to the pressure the car maker specifies, and measure it on cold tyres: in the morning, before driving. The plate in the door opening or on the fuel filler flap gives precisely the “cold” pressure. Reset an indirect system after every top-up (the procedure is in the car’s handbook).

Sensors and the second set of wheels

With an indirect system it is simple: change the wheels, inflate, reset. With a direct one the sensors live in the wheels, so in winter there are three options:

• A second set of sensors in the winter wheels is the most convenient and the most expensive, and on some cars they also have to be “registered” with a diagnostic tool.
• Moving the sensors from the summer wheels to the winter ones is cheaper, but twice a year it means extra tyre removal and a risk to the sensor.
• Winter wheels without sensors are the cheapest, but the fault light will stay on all winter, just when the pressure jumps most.

It is a matter of money and convenience, so decide for yourself. Just remember that in EU countries a clearly non-working system is a serious defect at the periodic inspection. And a warning light that stays on all winter no longer warns about anything, and the pressure gauge becomes your companion again.

Another expense: a sensor battery lasts roughly 5-10 years, and it is usually replaced together with the sensor. The sealant from a repair kit can clog the sensor’s port (more in the article on life without a spare wheel). After rotating the wheels an indirect system must be reset, and on some cars a direct one has to “learn” which sensor is now where.

What should you demand from the tyre fitter?

A pressure sensor is a delicate thing, and a tyre changer is not. A mechanic friend of mine says that a fitter who has once paid out of his own pocket for a crushed sensor never again presses the bead breaker near the valve. But he does not have to learn on your wheel, so warn him about the sensors and insist on this:

• Break the bead only on the side opposite the valve (on both sides of the wheel), and when removing the tyre set the mounting head about 15 cm behind the valve.
• Lubricate the bead with fitting spray or a soapy solution, not paste (it can clog the sensor’s filter).
• Replace the sensor’s valve seal and core when changing tyres, if the maker requires it (rubber ages and starts to leak air).
• Check after fitting that the car sees all the sensors.

If these demands surprise the fitter, look for another tyre fitter.

To sum up: a tyre pressure monitoring system is one of the few electronic aids that really make driving safer. But like ABS, it makes neither the gauge nor your head redundant: the light comes on when a fifth of the pressure is already missing, or more. If the light comes on while you are driving, slow down, stop in a safe place and inspect the wheels! Not “home isn’t far”, and not “I’ll look in the morning”.

A warning light has never yet inflated a single wheel.

All about tyre pressure