TPMS Sensors Explained: The Tyre Pressure Warning Light

Published 2026-01-02 · By Abed Jabbarkhel, Owner, Park Royal Tyre & Alignment Centre

That little horseshoe-shaped symbol with an exclamation mark inside it catches plenty of drivers off guard. It is the tyre pressure warning light, and it is driven by a system called TPMS. Knowing how it works, why it triggers, why a faulty sensor can fail your MOT and what needs doing when you change tyres will keep you safer and save you money. Here is a clear, jargon-free guide from our workshop in Park Royal.

What is a TPMS and what does it do?

A Tyre Pressure Monitoring System, or TPMS, is an electronic safety feature that constantly checks the air pressure in your tyres and warns you on the dashboard the moment it drops too low. It has been fitted as standard on most new cars since 2012, because under-inflated tyres wear faster, burn more fuel and are far more likely to fail (Source).

The system exists because human eyes are poor at spotting low pressure. A tyre can lose a significant amount of air before it visibly looks flat, and by then it is already overheating and shedding tread. TPMS removes the guesswork by alerting you early, so a small problem gets fixed before it becomes a blowout or expensive, uneven tyre wear.

Correct pressure also matters for stopping distances, grip in the wet and steering response, so a warning light is worth acting on quickly rather than leaving for weeks. Think of TPMS as an early-warning gauge rather than a nuisance: the few minutes it takes to check and reinflate a tyre can prevent a roadside breakdown or a much larger repair bill later on.

Direct vs indirect TPMS: what is the difference?

There are two designs, and they behave quite differently. Direct TPMS uses a battery-powered sensor mounted inside each wheel that measures the actual pressure and radios it to the car. Indirect TPMS has no sensors at all; it uses the ABS wheel-speed sensors to detect when an under-inflated tyre rotates at a slightly different rate to the others.

Direct systems

Direct sensors give accurate, per-wheel readings, and many cars display the exact pressure for each tyre on the dashboard. The trade-off is cost and maintenance. Each sensor contains a sealed battery, and impacts with kerbs and potholes or general wear can damage the sensors so they need replacing. They also need reprogramming when they are renewed.

Indirect systems

Indirect systems are cheaper because they reuse the vehicle's existing ABS hardware rather than adding four separate sensors. The catch is that they must be reset every time air is added or the tyres are rotated to stay accurate, and they can be slower to notice a gradual, even loss of pressure across all four wheels at once (Source).

Why is my tyre pressure warning light on?

The light usually comes on when at least one tyre is around 25% or more below its recommended pressure, the point at which the system flags a genuine safety risk to you and other road users. Cold weather, a slow puncture, a leaking valve or a dead sensor battery can all be behind it (Source).

There is a useful difference to watch for. A steady light usually points to genuine low pressure, so check and adjust your tyres first. A light that flashes for around 60 to 90 seconds when you start the car and then stays on often signals a system fault, such as a failed sensor, rather than a pressure problem. If the light will not clear after you have corrected the pressures, the cause is almost always the sensor or the system itself.

Is a faulty TPMS an MOT failure?

Yes. Since 1 January 2015, any car first used on or after 1 January 2012 must have a working TPMS to pass its MOT. A tester will record a major defect and fail the vehicle if the system is "malfunctioning or obviously not working", according to the DVSA MOT inspection manual (Source).

There is an important nuance. The MOT fail is for a system malfunction, not simply for a light showing low pressure. If the warning lamp is on because a tyre is genuinely under-inflated, the TPMS is doing its job and there is no fault to record. TyreSafe confirms that from 1 January 2015 an inoperative or faulty TPMS results in an MOT failure for affected vehicles, which is why a dead sensor can quietly cost you a retest (Source).

Because the rule applies from the date the car was first used, most cars on UK roads today fall within scope. If you are booking an MOT and the tyre pressure light has been glowing for weeks, it is worth getting the cause diagnosed beforehand so a simple sensor issue does not turn into a failed test and a wasted trip.

How long do TPMS sensor batteries last?

Direct TPMS sensor batteries are sealed inside each sensor and typically last up to ten years, according to the RAC. They cannot be swapped out on their own, so when the battery finally dies the whole sensor has to be renewed. This is exactly why older cars sometimes show a persistent fault light with no genuine pressure problem at all (Source).

Battery life varies with use. Frequent short journeys, high mileage and extreme heat or cold all make the sensor transmit more often, which drains the cell faster, so five to ten years is a realistic real-world range. Indirect systems have no batteries to worry about, since they piggyback on the ABS, but they trade that convenience for less precise, per-wheel information.

Do TPMS sensors need servicing when you change tyres?

Yes, and it is the step many people overlook. Every time a tyre is removed from the wheel, the sensor's valve components are disturbed, so most fitters replace a small service kit, the valve core, seal, grommet and nut, for only a few pounds per wheel. Direct sensors then need reprogramming, and indirect systems must be reset through the dashboard.

Skipping this is a false economy. A perished valve seal can cause a slow leak that drains the very tyre the sensor is meant to protect, and a system left unreset may keep the warning light on. When you have new tyres fitted or a puncture repaired, it is always worth asking whether the TPMS service kits and a reset are included in the price rather than added on afterwards.

If you are choosing a new set of tyres, the same principle applies. Factor the sensor service into the job from the start so your car drives away with the warning light cleared and the system reading accurately, ready for its next MOT.

Frequently asked questions

Can I keep driving with the TPMS light on?

You can drive, but you should not ignore it. A steady light usually means a tyre is low, which raises the risk of a blowout and increases fuel use, so check and adjust your pressures promptly. A flashing light that then stays on points to a system fault that needs diagnosing, and on an affected car it can also mean an MOT failure.

Does topping up the air always turn the light off?

Not always. On direct systems the light usually clears within a short drive once the pressures are correct. Indirect systems normally need a manual reset through the dashboard menu after inflation. If the light stays on after you have set every tyre correctly, the cause is almost certainly a sensor or system fault rather than low pressure, and it needs a proper diagnostic check.

Are TPMS sensors a legal requirement in the UK?

Sensors must be fitted and working on cars that were originally built with TPMS, which covers cars first used on or after 1 January 2012. You cannot legally remove or disable a working system, and since 1 January 2015 a malfunctioning one is a major MOT defect that can leave an eligible vehicle unable to pass its test.

How much does a replacement TPMS sensor cost?

Costs vary by make and model, but a replacement sensor is usually far cheaper than drivers expect, and we stock both vehicle-specific and programmable universal sensors to suit most cars. The valve service kit fitted during a tyre change is only a few pounds per wheel. Call us on 020 3886 2355 for a quick, no-obligation price for your vehicle.

Why is my TPMS light on when my tyres look fine?

A tyre can be 25% under-inflated and still look normal to the eye, so the light may be catching a genuine slow leak before it becomes visible. Equally, on an older car the cause is often a sensor whose battery has reached the end of its life. Either way, an electronic diagnostic check will quickly tell you which it is.

If your tyre pressure warning light is on, or you are due new tyres and want the TPMS handled properly, Park Royal Tyre & Alignment Centre is here to help. As an RAC and AA approved garage in Park Royal, London NW10 7TR, we read sensor IDs, programme replacements and reset both direct and indirect systems so the light clears correctly. Call us on 020 3886 2355, message us on WhatsApp at 07476 586 589, or book online here.