What the light looks like
The tire pressure monitoring light is typically an amber symbol that resembles a cross-section of a tire, with an exclamation mark in the middle. It may be a flat-bottomed horseshoe shape. On many cars, it appears when the monitoring system detects that one or more tires are significantly below the recommended pressure. In many countries, the system is required on newer passenger cars, though the details differ by region.
Low pressure matters because under-inflated tires wear unevenly, can overheat, reduce handling and braking performance, and increase fuel use. In the worst case, a tire can fail.
How monitoring systems work
There are two main types.
Direct systems use a small sensor inside each wheel, usually attached to the valve stem or the wheel rim. The sensor measures the air pressure and sends the reading by radio to the car’s computer. Some cars can display the pressure of each tire individually. The sensors typically have batteries that eventually run out, often after several years, so they need replacement at some point.
Indirect systems use the car’s anti-lock braking sensors rather than a pressure sensor in the tire. A tire with low pressure has a slightly smaller rolling diameter, so it turns a little faster than the others. The computer notices the difference and triggers the light. Indirect systems usually need to be reset after you adjust the pressure, and they can be less precise.
Your owner’s manual says which type you have.
Why the light comes on
- A real loss of pressure. The tire may be low from normal slow leakage, which is common, or from a puncture, a leaking valve or a damaged wheel.
- Cold weather. Air pressure drops as temperature falls, so the light often appears on the first cold morning of fall. Pressure typically falls by a small amount for each drop in temperature, so a tire that was only slightly low in summer can cross the threshold.
- A recent tire change or rotation. Sensors may need to be reset or relearned, particularly after rotation, or after fitting a spare or winter wheels.
- A faulty sensor or low sensor battery. This usually produces a flashing light.
- A different tire size or a mismatch, especially with indirect systems.
Steady versus flashing
On many cars, a steady light means one or more tires are low. A light that flashes for about a minute at startup and then stays on commonly indicates a fault in the monitoring system itself, such as a dead sensor battery or a missing sensor. In that case the system may not warn you about actual low pressure, so check the tires manually. Your owner’s manual explains the exact behavior of your car.
What to do when it comes on
- Don’t ignore it, but don’t panic. If the car is handling normally, you can usually drive carefully to a place where you can check the tires. If the car pulls, feels unstable or you hear flapping, stop safely.
- Look at all four tires. A visibly flat or sagging tire needs attention straight away. Also look for a nail or screw in the tread.
- Check the pressure with a gauge, ideally when the tires are cold, meaning the car has been parked for a few hours or driven only a short distance. Checking tire pressure at home takes only a few minutes with a hand gauge.
- Find the recommended pressure. It is usually on a sticker in the driver’s door jamb or in the manual, not the maximum figure printed on the tire sidewall.
- Inflate to the recommended pressure using a pump at home or at a service station. If the pressure was extremely low, the tire may have been driven flat, and it might be damaged even after reinflation, so have it inspected.
- Check the spare, if your car has one, since it may also be low.
- Drive a short distance. On many cars, the light goes out after a few minutes of driving. Some cars need a manual reset using a button or a menu setting, so check the manual.
- If the light returns, or one tire loses air repeatedly, you may have a slow puncture, a leaking valve or a damaged wheel. Have the tire checked by a professional. Repairs for a slow puncture are often simple and inexpensive, but they should be done properly.
If a tire is flat, follow the steps for changing a flat, or consider the spare tire and sealant kit your car carries. Some cars have run-flat tires, which can be driven for a limited distance at reduced speed after losing pressure, but follow the manufacturer’s limits and don’t treat them as normal tires.
Don’t rely on the light alone
The light usually comes on only when pressure falls well below the recommended level, and often 20 to 25 percent low is the kind of threshold used, though this varies. That means a tire can be underinflated enough to affect wear and efficiency before the light appears. A monthly manual check is still a good habit, and it is cheap protection for tread depth and safety. Rotation and wheel alignment also affect how evenly tires wear, so a regular check helps you notice problems early.
When to get professional help
See a tire shop or mechanic if:
- The light flashes at startup and does not go away
- One tire keeps losing pressure
- You find damage, a bulge or a nail in a tire
- The light stays on after correct inflation and a reset
- You are unsure whether a tire is safe to drive on
Replacing a sensor or resetting the system is a routine job for most shops. Costs vary, so ask for an estimate.
Seasonal habits
Check pressures when temperatures change a lot, before long trips, and when carrying heavy loads. If you swap between summer and winter wheels, make sure the sensors are compatible with the car’s system, or ask the shop to program them.