What is a TPMS (Tyre pressure monitoring system), and how does it work?

You’re taking in the scenery on your morning commute, and then you see it out of the corner of your eye – the tyre pressure monitoring system (TPMS) warning light. Modern cars are fitted with a TPMS to let you know when your tyres are running low on air pressure. So, what should you do about it to stay safe on the road? We’ll explain everything you need to know about the TPMS and how tyre pressure sensors work.

What is a TPMS?

A TPMS is a tyre pressure monitoring system. These pressure sensors check the level of air in your tyres and notify you of a pressure drop, or if your tyres become under-inflated. The TPMS helps alert you to pressure issues so you can avoid driving unsafely. Being aware of a drop in tyre pressure levels sooner rather than later also significantly lowers the risk of tyre failure and ensures your tyres last as long as possible.

How do tyre pressure sensors work?

Tyre pressure sensors work by consistently taking readings of your tyre’s air levels. There are two types of tyre pressure systems: direct and indirect. Although they both use sensors, they work differently.

Type 1 - Direct tyre pressure monitoring system

A direct TPMS measures tyre pressure reading from sensors inside the tyre valve. These readings are then sent to your vehicle’s computer. If there’s an issue, it creates a dashboard alert – which is why you see the warning light near your steering wheel.

Usually, a direct TPMS sends an alert if air pressure drops by 25% or more. In newer cars, it may tell you which tyre is low in pressure on the dashboard, so you know which one needs air. The TPMS may also notify you of what the air pressure is and what it needs to be, which makes it easier when you get to your nearest petrol station.

Other tyre monitoring systems simply let you know something is wrong with any of the four tyres, which means you’ll need to check them manually. You may also need to dig out the car handbook to see what the PSI level should be – or do a quick online search.

Direct TPMS are the most common form of sensor because they’re more accurate. They take readings directly from the tyre valve in each tyre, leaving less room for error. That said, they can be easily damaged when removing and fitting new tyres, which can cause them to detect non-existent problems.

Type 2 - Indirect tyre pressure monitoring system

An indirect TPMS relies on wheel speed sensors, rather than air pressure sensors. They detect a drop in pressure through the anti-lock braking system (ABS), which measures the tyre’s speed and revolution. Essentially, a flat tyre will spin faster and make more turns (revolutions). That means these sensors look for an increase in both speed and revolution to alert you to a tyre pressure problem.

In general, indirect TPMS are less accurate because they assess speed rather than air pressure – even though they warn you of a drop in pressure. If you fit a new tyre with a slightly different size, it may cause the system to flag a drop in pressure despite having a fully inflated tyre.

Why is the TPMS important?

Any warning light on a vehicle is important. Even though air pressure might not be something critical like an engine problem, that doesn’t mean you should ignore it. Failing to inflate your tyres can easily become a critical problem. After all, incorrect or underinflated tyres can cause unsafe driving, which may lead to accidents.

Before TPMS technology, vehicles couldn’t inform you of a drop in tyre pressure, which means you needed to check every tyre manually – and remember to do so regularly. Now, sensors let you know when there’s an issue so you can rectify the problem before it causes further damage to your tyres. That doesn’t mean you shouldn’t check your tyre pressure manually, but it’s good to know you can rely on the car to inform you of an issue.

What to do if the TPMS indicator light turns on

If the tyre pressure sensor light flashes up on the dashboard, you should pull over, but only when it is safe to do so. Find somewhere safe to stop and turn off your engine. You need to know how to check your tyre pressures, so you can add air where needed.

If it’s safe to drive, take your vehicle to the nearest petrol station or garage to check the pressure using a gauge machine. You’ll need to set the PSI level to ensure the tyre has enough air. To find the correct PSI, either look on your car dashboard or find the vehicle handbook.

Remember, low air pressure can also mean a slow puncture. If this is the case, head to your local Halfords garage to get the tyre checked and replaced. We’ll fit new tyres for you there and then, so you can get back on your way. If your tyre is completely flat or it’s not safe to drive, don’t worry. We’ll send one of our tyre technicians to you as part of our puncture repair service, so you can safely get your vehicle to one of our garages.

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Did you know that we now offer same day tyre fitting service across our garages nationwide? Simply book online before 1pm and we can replace your tyres the same day to help you get you back on the road safely and swiftly. Please note that availability varies by location. For more information, please head on over to our dedicated same day tyre fitting page where you can find answers to frequently asked questions, choose the right tyres for your vehicle and book your appointment.

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