Tire pressure (TPMS) light

The tire pressure light (the TPMS light, or "tyre pressure light" in the UK) looks like a cross-section of a tire: a horseshoe shape with an exclamation mark inside. TPMS stands for tire pressure monitoring system. New cars have needed one in the US since 2007 and in the EU since November 2014, so most cars on the road today have it.

It is not a "stop now" light, but it is a safety warning. Mazda's owner's manual, for example, warns that braking and steering get worse with low tires and that a tire can burst.

What the light is telling you

• Steady light: at least one tire is well below its correct pressure. US rules require the light to come on within 20 minutes once a tire is 25% or more below the recommended cold pressure. For a tire that should be at 35 psi (2.4 bar), that means about 26 psi (1.8 bar). In AAA's tests, most cars warned at around 70–75% of the correct pressure, so by the time the light is on, the tire is already quite low.
• Flashing for about a minute, then steady: the system has a fault, such as a failed sensor or a sensor battery running low. US rules require this flash to last 60 to 90 seconds. Your tires may be fine, but the system can no longer warn you, so check them by hand and get it repaired.
• Some cars show each tire's pressure on the dashboard, which tells you straight away which tire is low.

What to do

1. At the next safe stop, look at all four tires. If one is flat, bulging, cut or has something stuck in it, do not drive on it. Fit the spare, use the repair kit, or call for help.
2. Find the correct pressure on the sticker inside the driver's door frame, inside the fuel filler flap, or in your owner's manual. It can be different front and rear, and some cars list a higher figure for a full load.
3. Check all four with a gauge and inflate to the right figure. Readings are most accurate when the tires are cold, before you have driven far.
4. Drive for a few minutes to let the light go out, or reset the system if your owner's manual says so.
5. If the light comes back within days, a tire is probably losing air: have it checked for a slow puncture.

Likely causes, most common first

1. Cold weather

   Air contracts when it is cold, so pressure drops on the first cold mornings of the year. Tires that were slightly low can cross the warning line overnight. Topping them up is the fix.

2. Slow puncture or leaking valve

   A nail, a damaged rim edge or a leaking valve lets air out slowly over days. A tire shop can find the leak and usually repair it.

3. Normal air loss over time

   All tires lose a little air through the rubber. If nobody has checked them for months, they can drift low enough to set off the light.

4. Faulty TPMS sensor

   Many cars use a battery-powered sensor inside each wheel. Toyota says its sensor batteries last around ten years and cannot be replaced, so the whole sensor is changed. A failed sensor makes the light flash at start-up.

5. Spare wheel or new wheels without sensors

   If a wheel has no sensor, the system cannot see it and the light stays on.

What it costs to fix

Inflating your tires costs little or nothing at most fuel stations. A puncture repair at a tire shop is usually a small bill. Sensors cost more:

DIY or garage?

How to prevent it

• Check all four tires, plus the spare if you have one, once a month with a gauge. AAA recommends this because a tire can be more than 20% low before the light comes on.
• Check when the tires are cold, and use the pressures on the door sticker.
• Top up when the weather turns cold.
• Look at your tires when you fill up: nails, cuts and bulges are easier to catch early. See how long tires last.