What Your Tyre Pressure Warning Light Means (and What to Do)

What the light actually means

The TPMS warning light usually means one or more of your tyres has dropped below the recommended pressure. The system exists to improve safety, fuel economy, and tyre life by alerting you before a soft tyre becomes a dangerous one. On most cars the same amber light also flashes for a few seconds and then stays on when there is a fault with the monitoring system itself, rather than the tyres.

There are two types of system. Direct TPMS uses a sensor inside each wheel to measure actual pressure, while indirect TPMS works through the ABS wheel-speed sensors, detecting when one tyre is rotating slightly faster because it is underinflated.

Common reasons it comes on

• A slow puncture from a nail, screw, or debris letting air out gradually
• A leaking or corroded valve
• A drop in outside temperature (pressure falls roughly 1-2 PSI for every 10C colder)
• Natural pressure loss over weeks and months
• Wheel or tyre damage from a pothole or kerb
• A faulty or flat sensor battery on a direct TPMS system

What to do when it appears

As soon as it is safe, check all four tyre pressures with a reliable gauge and compare them to the figures on the sticker inside your driver door shut or in the handbook. Inflate any low tyre to the correct pressure. If the light was triggered simply by underinflation, it will usually go out on its own after a short drive, or after you reset the system following the handbook instructions.

If the light comes back on soon after you have set the pressures correctly, you very likely have a slow puncture or a leaking valve and the tyre should be inspected. If it stays on even though every tyre is at the right pressure, the system itself may have a fault, such as a dead sensor battery.

Does it affect your MOT?

Yes. All cars registered from January 2012 must have a working TPMS, and it is checked during the MOT. If the warning light is illuminated because of a system fault when the car is tested, it is an automatic MOT failure. A light that is on simply because a tyre is soft can be cleared by correcting the pressure before the test.

This is why you should never cover up, disable, or ignore the light. Get the cause fixed properly so both your safety and your MOT are covered.

When to call a professional

If the light keeps returning, if a tyre will not hold pressure, or if you suspect a puncture or a faulty sensor, have it checked promptly. A mobile tyre fitter can come to your home or workplace, find the leak, replace a faulty valve or sensor, and repair or replace the tyre on the spot, so a warning light never turns into a roadside breakdown.