TPMS Warning Light On? What It Means and What to Do
If the amber horseshoe-shaped symbol with an exclamation mark has lit up on your dashboard, your car is alerting you to an issue with its Tyre Pressure Monitoring System (TPMS). This warning should never be ignored, as it directly impacts your safety, tyre longevity, and fuel economy. In this comprehensive guide, we explain exactly what the TPMS warning light UK system is, the difference between direct and indirect systems, what triggers the light, step-by-step instructions on what to do when it appears, and when it indicates a faulty sensor rather than just low pressure.
What is TPMS?
The Tyre Pressure Monitoring System (TPMS) monitors tyre pressure to warn you of underinflation, helping to prevent accidents, reduce tyre wear, and improve fuel efficiency. In the UK, TPMS became mandatory for all new passenger cars sold after November 2014.
There are two primary types of Tyre Pressure Monitoring Systems used by vehicle manufacturers today:
- Direct TPMS: This system uses physical, battery-powered pressure sensors mounted inside each wheel assembly, typically attached to the tyre valve. These sensors measure the actual air pressure and temperature inside the tyre and transmit this data wirelessly to the vehicle's engine control unit (ECU). Direct TPMS is highly accurate but the sensors have a limited battery life.
- Indirect TPMS: Instead of physical sensors, this system relies on the car's existing Anti-lock Braking System (ABS) wheel speed sensors. An underinflated tyre has a slightly smaller outer diameter and rotates faster than a correctly inflated tyre. The ABS sensors detect this rotational speed difference and trigger the TPMS dashboard warning. Indirect systems must be manually reset after adjusting tyre pressures.
What does the TPMS warning light look like?
The TPMS warning light is an internationally standardised dashboard symbol. It is yellow or amber in colour and resembles a cross-section of a tyre — shaped like a horseshoe with flat bottom tread lines — containing an exclamation mark in the centre. The light can indicate two distinct situations:
- The light stays on solid: If the warning light remains solid while you are driving, it means that the system has detected that one or more of your tyres is 25% or more below the recommended cold inflation pressure. This is a direct warning that your tyres need air immediately.
- The light flashes, then stays solid: If the light flashes for 60 to 90 seconds when you first turn the ignition and then remains illuminated, this indicates a system fault. The TPMS is not actively monitoring your tyres because it has detected an internal error, such as a dead sensor battery or failed sensor.
What to do when the TPMS light comes on
If your TPMS dashboard warning light illuminates while driving, do not panic. Follow these steps to resolve the issue safely:
Step 1: Check your tyre pressures manually
Find a safe place to pull over, or drive to the nearest petrol station. Let the tyres cool down for a few minutes if you have been driving at speed. Use a manual tyre pressure gauge or a digital forecourt pump to check the pressure of all four tyres (and the spare, if applicable).
Step 2: Locate the correct tyre pressure values
Compare your measured tyre pressures to the recommended cold inflation pressures. You can find these specifications on a placard inside the driver's door frame, on the inside of the fuel filler cap, or within your vehicle owner's manual. Do not use the pressure written on the sidewall of the tyre itself, as this is the maximum pressure, not the recommended running pressure.
Step 3: Inflate the tyres and reset the system
Inflate any underinflated tyres to the exact recommended pressure. Once all tyres are correctly inflated, drive your vehicle at a steady speed (above 20 mph) for a few miles. Many modern direct TPMS systems will register the new pressures and turn off the warning light automatically within 10 to 15 minutes. However, if your car uses an indirect TPMS system, or if the light remains on, you must perform a manual reset. This is usually done through your vehicle's instrument cluster menu or by pressing a dedicated TPMS button.
Why is my TPMS light on even though my tyres look fine?
It is very common for drivers to inspect their tyres visually after the TPMS light comes on and conclude that everything is fine because no tyre looks flat. However, visual inspection is unreliable for several reasons:
- Visual inflation limits: Modern radial tyres have stiff sidewalls. A tyre can lose up to 30% of its air pressure before it begins to look visually underinflated or bulges at the bottom. By the time a tyre looks flat to the naked eye, it is already dangerously underinflated.
- Temperature drops: Cold weather causes air to contract. For every 5°C (10°F) drop in ambient temperature, your tyre pressure will drop by approximately 1 PSI. During winter months, you may find that the TPMS light comes on first thing in the morning when the air is cold, but turns off after you have been driving for a while and the tyres have warmed up. You should still check and adjust the pressures when the tyres are cold.
- Slow punctures: A small nail, screw, or piece of glass embedded in the tread can create a slow leak that releases only 1 or 2 PSI per day. While the tyre might look fine today, the TPMS will detect the cumulative pressure loss over a few days and trigger the light.
When the light means a sensor fault (flashing then solid)
If your TPMS light flashes before staying on solid, it indicates a hardware or system fault rather than an inflation problem. In direct TPMS systems, each wheel sensor is powered by an internal lithium-ion battery. These batteries are sealed within the sensor and cannot be charged or replaced. The lifespan of a TPMS sensor battery is typically between 5 and 10 years, or around 100,000 miles. When a sensor battery dies, the sensor stops transmitting and the vehicle's ECU registers a system fault.
When a TPMS sensor fails or the battery runs out, the entire sensor unit must be replaced. A new sensor must be installed inside the wheel, the wheel must be re-balanced, and the new sensor's unique ID code must be programmed to the car's ECU so they can communicate. Replacing a TPMS sensor in the UK typically costs between £50 and £120 per wheel, which includes the sensor part, professional fitting, wheel balancing, and reprogramming.
Can I ignore the TPMS warning light?
Ignoring a TPMS warning light is highly discouraged and can lead to serious consequences. Driving on underinflated tyres is dangerous and expensive for several key reasons:
| Risk Factor | Impact of Underinflation |
|---|---|
| Braking Distance | Low tyre pressure increases the stopping distance of your vehicle, particularly on wet roads, raising the risk of collisions. |
| Tyre Wear | Underinflated tyres wear rapidly on the outer edges of the tread, significantly shortening the lifespan of the tyre and leading to premature replacement costs. |
| Fuel Economy | Soft tyres have higher rolling resistance, requiring the engine to work harder. This can reduce your fuel economy by up to 3% to 5%. |
| Handling & Safety | Low pressure compromises the vehicle's handling, making cornering unpredictable. Severe underinflation can lead to overheating, tyre delamination, and sudden blowouts at motorway speeds. |
| MOT Status | An active TPMS warning light or a known system fault is classified as a major defect under UK MOT testing rules. If your car was registered after November 2014, it will fail its MOT test immediately if the TPMS light is active. |
Need a TPMS Check or Sensor Replacement in Ashford?
If your TPMS light won't go out, or you suspect a faulty sensor, bring your car to our garage at 32-34 Feltham Road, Ashford (TW15 1DH). We will test your tyre pressures, check for slow punctures, inspect the sensors, and provide honest advice with no obligation.