What a tyre pressure monitor does
A tyre pressure monitoring system (TPMS) is a sensor in each tyre that watches the air pressure and sends a warning to your dashboard when pressure drops below a safe level. Most cars sold since 2015 have this as standard. The system does not inflate your tyres or fix a leak—it only tells you something is wrong so you can stop and check before the tyre fails.
When pressure falls, the sensor triggers a warning light on your dashboard, usually a picture of a tyre with an exclamation mark. Some cars also show the actual pressure reading on the instrument cluster or infotainment screen. The warning appears because low pressure changes how the tyre behaves: it heats up faster, wears unevenly, reduces fuel economy, and can fail suddenly if you keep driving.
TPMS exists because tyre failure is a real safety risk. A flat or severely underinflated tyre can cause loss of control, especially at motorway speed or in wet conditions. The system gives you time to pull over safely rather than discovering the problem when the tyre bursts.
Key Takeaways
- TPMS sensors monitor air pressure in each tyre and alert you via a dashboard light when pressure drops, usually by 25% below the manufacturer's recommended level.
- The warning light does not mean your tyre is flat—it means pressure has fallen enough to affect safety and fuel economy, and you should check and inflate soon.
- Sensors are powered by small batteries that last 5 to 10 years, after which the sensor must be replaced as a unit; they cannot be repaired or have the battery changed separately.
- Cold weather naturally lowers tyre pressure by roughly 1 psi for every 5°C drop in temperature, which can trigger the warning even if there is no leak.
- A faulty TPMS sensor or receiver module will show a warning light that does not clear after you inflate the tyres, and requires diagnostic testing to identify which component has failed.
How TPMS sensors detect pressure changes
Each tyre contains a small wireless sensor mounted inside the wheel rim, usually attached to the valve stem or the wheel itself. The sensor measures air pressure continuously and transmits the reading to a receiver module in your car, typically located near the engine or under the dashboard. When pressure drops below the threshold—usually about 25% below the manufacturer's recommended pressure—the receiver triggers the warning light.
The threshold is not arbitrary. Tyre manufacturers set a safe minimum pressure for each vehicle, printed on a sticker inside the driver's door frame or in your handbook. If your car's recommended pressure is 32 psi, for example, the warning will trigger around 24 psi. This gives you a safety margin: the tyre is still usable, but pressure has fallen far enough that you need to act.
The sensor does not measure how much pressure you have lost or how fast it is leaking. It only reports whether pressure is above or below the threshold. This is why the warning light comes on the same way whether you have a slow puncture or a rapid leak—the system is binary, not graduated.
Why the warning light comes on in cold weather
One of the most common reasons drivers see the TPMS warning is a sudden drop in temperature. Air contracts when it cools, so tyre pressure naturally falls as the weather gets colder. The relationship is roughly 1 psi of pressure lost for every 5°C drop in ambient temperature. If you park your car overnight in winter and the temperature falls 10°C, you may lose 2 psi without any leak at all.
This is not a malfunction—it is physics. Your tyres are fine; the pressure has straightforward dropped into the warning zone. Once you drive and the tyres warm up from friction, pressure rises again and the light usually goes out. However, if the light stays on after the car has warmed up, or if it comes back repeatedly, there is likely a slow leak that needs investigating.
To avoid false alarms in winter, check your tyre pressure when the tyres are cold (before driving or at least three hours after driving) and inflate to the manufacturer's recommended level. This gives you a buffer so that normal temperature swings do not trigger the warning.
Sensor batteries and replacement costs
TPMS sensors are powered by small batteries sealed inside the sensor unit. These batteries typically last between 5 and 10 years, depending on how often the sensor transmits and the quality of the battery. Once the battery dies, the sensor stops working and must be replaced as a complete unit—the battery cannot be changed separately.
Replacement cost varies widely. A single sensor typically costs between £30 and £100 depending on your car's make and model, plus fitting labour. Some sensors are more expensive because they are integrated with the valve stem or require programming to your car's receiver module. If all four sensors need replacing at once—which can happen if they were all installed at the same time—the total bill can be £200 to £400 or more.
Many garages recommend replacing sensors in pairs or all four at once when one fails, because the others are likely to fail soon after. This is not always necessary, but it can be cost-effective if you want to avoid multiple visits. Ask your garage whether they recommend replacing just the failed sensor or all four.
What to do when the TPMS warning light appears
The first step is to check your tyre pressure as soon as safely possible. Pull over at a petrol station or garage and use an air pump to measure the pressure in all four tyres. Compare the readings to the manufacturer's recommended pressure, printed on the door frame sticker or in your handbook. If any tyre is below the recommended level, inflate it to the correct pressure.
After inflating, drive for a few minutes to let the tyres warm up, then check whether the warning light has gone out. If it has, the problem was low pressure and you have solved it. If the light stays on or comes back within a few days, there is likely a slow leak or a faulty sensor. A slow leak may be caused by a puncture, a damaged valve stem, or a bead leak where the tyre does not seal properly to the rim.
Do not ignore a persistent TPMS warning. Continuing to drive on underinflated tyres increases the risk of tyre failure, reduces fuel economy, and accelerates uneven wear. If you cannot find a leak by visual inspection, take the car to a garage for a pressure test and leak detection. They can identify whether the problem is a puncture, a faulty sensor, or a receiver module fault.
Faulty sensors and receiver module problems
Sometimes the TPMS warning light comes on even after you have inflated all four tyres to the correct pressure and driven for several minutes. This usually means either a sensor has failed or the receiver module is not working properly. A faulty sensor will not transmit pressure data, so the receiver cannot confirm that pressure is safe. A faulty receiver will not receive signals from the sensors, even if they are working.
Diagnosing which component has failed requires specialist equipment. A garage will use a TPMS diagnostic tool to check whether each sensor is transmitting and whether the receiver is receiving. They can also check the sensor battery voltage to see whether it is running low. Once they identify the faulty component, it can be replaced.
If the warning light flashes rather than staying on, this usually indicates a receiver module fault rather than a sensor problem. A flashing light means the system itself is not working properly and needs attention sooner rather than later, because you have no pressure monitoring until it is repaired.
Seasonal tyre pressure changes and maintenance
Because temperature affects pressure, it is good practice to check your tyre pressure twice a year—once in autumn as temperatures drop, and once in spring as they rise. This helps you stay ahead of seasonal pressure changes and reduces the chance of the TPMS warning appearing unexpectedly.
When you check pressure, always do it when the tyres are cold: either before you have driven that day, or at least three hours after driving. Pressure measured on a warm tyre will be higher than the actual resting pressure and will give you a false reading. Inflate to the manufacturer's recommended pressure, not the maximum pressure printed on the tyre sidewall—the sidewall number is the absolute limit, not the target.
If you replace your tyres, the new tyres will have new sensors already fitted (or the old sensors will be transferred to the new wheels). If you fit winter tyres on a separate set of wheels, each set will have its own sensors, and you may need to reprogram the receiver to recognise the winter sensor IDs when you swap wheels. Your garage can do this during the tyre change.
Frequently Asked Questions
Can I drive with the TPMS warning light on?
You can drive to a garage or petrol station to check and inflate your tyres, but do not ignore the warning or drive long distances with it on. Low pressure increases the risk of tyre failure, especially at motorway speed or in hot weather. If the light stays on after you have inflated the tyres, there is a leak or sensor fault that needs investigation.
Why does my TPMS light come on only in winter?
Cold air contracts, so tyre pressure naturally falls as temperature drops. A 10°C temperature fall can reduce pressure by 2 psi. If your tyres are already close to the warning threshold, winter cold can push them over it. Check pressure when tyres are cold and inflate to the manufacturer's recommended level to prevent this.
Do I need to replace all four TPMS sensors at once?
No, you can replace just the faulty sensor. However, if all four sensors were installed at the same time, they are likely to fail around the same time, so some garages recommend replacing them in pairs or all four together to avoid multiple visits. Ask your garage for their recommendation based on your car's age and sensor history.
What is the difference between the TPMS warning light and a low-pressure warning?
They are the same thing. The TPMS warning light indicates that at least one tyre has pressure below the safe threshold. Some cars show which tyre is low on the dashboard display, while others just show the warning light. Either way, you need to check all four tyres and inflate as needed.
Can a TPMS sensor be repaired if the battery dies?
No. TPMS sensors are sealed units and the battery cannot be replaced separately. Once the battery dies, the entire sensor must be replaced. This is why sensor replacement is relatively expensive compared to other car parts.