It usually goes like this. You come back from the tire shop after swapping winter and summer tires, drive a kilometre or two, and suddenly a yellow horseshoe-shaped icon with an exclamation mark in the middle lights up on the dashboard. That's the TPMS light, the tire pressure monitoring system. It doesn't always mean a tire is punctured, but it does mean something in the system isn't right. Here's what we check first and why it's not smart to just ignore it.
What TPMS is and why newer vehicles have it
TPMS, or Tire Pressure Monitoring System, is a system that monitors the pressure in each tire and warns the driver when pressure drops below a safe level. The reason for introducing it is purely safety-related. Lower pressure increases fuel consumption, tires wear out faster and unevenly, and in critical situations, especially on the motorway, low pressure can lead to a tire blowout.
Since November 2014, the European Union has required TPMS on all new passenger vehicles rolling off the production line. That means almost every car from 2015 onwards has some form of this system. Older models may or may not have TPMS, depending on the trim level and market.
Direct TPMS in each wheel and indirect via ABS sensors
There are two technical solutions and it's important to know which one your car has, because both the failures and the fixes differ.
Direct TPMS has a physical sensor in each wheel, most often integrated into the tire valve or fastened to the rim. That sensor measures the actual air pressure and temperature inside the tire and sends the data by radio to the car's computer. It identifies itself with a unique ID, so the car knows exactly which wheel has lower pressure. Most somewhat more expensive and newer vehicles use this solution.
Indirect TPMS doesn't have sensors in the tires. Instead it uses the existing ABS wheel speed sensors (the anti-lock braking system). The computer tracks the rotation count of each wheel, and if one starts spinning slightly faster than the others, the system assumes its diameter has shrunk, meaning the tire has lost pressure. The system is cheaper for the manufacturer, but it requires a manual reset after every pressure change, tire rotation or seasonal tire swap.
The most common reasons the TPMS light comes on
These are the usual causes that bring drivers to us, roughly in order of frequency:
- An actual drop in pressure in one of the tires. This is the number one reason in our shop. The light is doing its job, you really do have lower pressure on one tire, probably a slow nail or a leaky valve.
- Cold weather. For every 10 degrees the temperature drops, pressure falls by about 0.1 bar. In the morning, below freezing, the light comes on, and by noon it goes away on its own. That's normal if the pressures were set in summer.
- Mechanical damage to the sensor during a tire change. On a direct TPMS, the valve and the sensor are one piece, and a tire fitter who isn't careful can crack the sensor body or damage the rubber seal.
- Worn-out battery in the sensor. The sensor has a factory-built lithium battery that can't be replaced separately.
- System not configured after a seasonal tire change. If the driver has two sets of rims with sensors, the car has to learn the IDs of the new set, which on some brands happens automatically while driving and on others requires programming.
- Fault on the module or antenna. Rarer, but the receiver in the car does sometimes fail.
What to do after swapping winter and summer tires
This is by far the most common situation we see when a driver shows up with the light on. It's very important to understand what the tire shop has to do and what you should check.
If the car has direct TPMS and only one set of rims, the sensors stay the same. The tire fitter needs to carefully dismount the tire so as not to damage the sensor, and during installation always use a new rubber seal and nut according to the manufacturer's specification and torque setting. The most common failures we see here are when someone uses ordinary valves meant for a car without TPMS, or when they over-torque the nut and the seal gives way.
If you have two complete sets of rims with their own sensors, the system needs to recognise the new set. Some cars do this themselves after a few dozen kilometres of driving above a certain speed. Others need an activation tool and a diagnostic scanner to write the IDs into the computer. If the car doesn't clear the light on its own after a day of driving, that's a sign programming is needed.
On indirect TPMS, the job is on the driver. After a tire change or pressure change, look for a reset button (often in the vehicle menu or on the console), press it so the system saves the current state as the reference, and you're done.
How long the sensor battery lasts
The battery in a direct sensor is lithium and is factory-sealed into the housing, it can't be replaced separately. Its lifespan is usually 5 to 10 years, depending on the climate, annual mileage and how often the system is activated. As the battery nears the end of its life, the sensor starts sending weaker or unreliable signals, and the car reads that as a fault and turns on the light.
When the battery dies, the whole sensor gets replaced. The replacement is usually done during a seasonal tire change, because the tire has to come off for the sensor to be removed. If your car is older than seven or eight years and the sensors have never been touched, it makes sense to plan on replacing the set at the next tire change. As for price, it depends on the sensor brand and on whether it's a universal or original part. The price depends on the condition of the car and the workshop, get in touch for an estimate.
Can you keep driving until you sort it out
A yellow TPMS light is not a reason to stop urgently on the motorway and call roadside assistance. But it is a reason to pull into the first petrol station and measure the pressure on all four wheels with a gauge. If the pressure matches the factory values (the values are on the sticker on the edge of the driver's door or on the fuel filler flap), the problem is probably in the sensor, not in the tire, and you can drive to the workshop normally.
If one tire is noticeably lower, top it up to the specified value and watch how it behaves. If it drops again in a few days, you have a slow leak, probably a nail or a problem with the valve, and it's off to the tire shop.
When a driver comes in with the TPMS light on, the first thing we do is measure the actual pressure on all four wheels with a gauge. In roughly 40 per cent of cases the sensor is working correctly and is simply reporting a real drop in pressure on one tire, most often due to a slowly leaking valve or a foreign object in the tread. Only once we've confirmed the pressures are fine do we move on to sensor diagnostics and, if needed, programming the system.
If the light has been on for more than one cold morning, or it's been going on and off all season, book an appointment and we'll check what's going on. It's safer to drive knowing the pressures are right than to hope the light will go away on its own.