What does a tire pressure sensor (TPMS) do?
TPMS stands for tire pressure monitoring system. What your car's tire pressure sensors do, where they are, how long they last, what a sensor fault means, and why you still need a gauge.
A tire pressure sensor warns you when a tire is low. When any tire drops about 25% below the pressure on your driver's door sticker, the sensor system turns on the yellow horseshoe-shaped light on your dashboard. Here's how it works, where it is, what can go wrong, and why it doesn't replace checking your tires yourself.
What does TPMS mean?
TPMS stands for tire pressure monitoring system: the sensors and the warning light together. "TPMS sensor" and "tire pressure sensor" mean the same thing. Since the 2008 model year, every new car and light truck sold in the US has to have one. Under the federal standard, FMVSS 138, the warning has to come on when a tire is 25% or more below the recommended pressure. For a car that calls for 35 PSI, that's around 26 PSI.
The two kinds of tire pressure sensors
Direct: a sensor inside each wheel
Most cars have a small sensor inside each wheel. It measures the pressure, often the temperature too, and sends it to the car by radio. If your dashboard shows the PSI of each tire, your car has this kind. Each sensor runs on a sealed battery that can't be swapped on its own; they're built to last about 5 to 10 years, and then the sensor is replaced.
Indirect: no sensors in the wheels
Some cars skip the wheel sensors and use the anti-lock brake sensors instead. A low tire is slightly smaller, so it spins a little faster than the others, and the car notices.
| Direct (sensor in each wheel) | Indirect (no wheel sensors) | |
|---|---|---|
| Shows each tire's PSI | Often | No |
| Needs a reset after adding air | Usually not | Yes |
| Has batteries that wear out | Yes, about 5 to 10 years | No |
| Notices all four tires dropping together (like in cold weather) | Yes | Less reliably |
| Needs service after new tires or a rotation | Sometimes (sensor relearn) | Reset only |
Where is the tire pressure sensor?
On cars with the direct kind, the sensor sits inside each wheel, attached to the inside end of the valve stem: the little stem with a cap where you add air. You can't see it without taking the tire off the wheel, which is why sensors usually get checked or replaced when you buy new tires.
Signs a sensor has a problem
- The tire pressure light blinks for about a minute when you start the car, then stays on.
- A message like "TPMS fault," "tire pressure sensor fault" or "service tire monitor system."
- One tire shows dashes instead of a number on the tire pressure screen.
- The light comes on even though all four tires check out fine with a gauge.
Common causes are a dead sensor battery, a sensor damaged by a pothole or during a tire change, a wheel without a sensor (like a spare or winter wheels), or sensors that need to be "relearned" after new tires or a rotation. Our guide to the tire pressure light covers what a blinking light means and how to reset the light after adding air.
Replacing a sensor
A tire shop can test which sensor has failed, replace it, and program it to your car. The price varies a lot by car and shop, so ask for a quote, and ask whether all four really need replacing or just the one that failed. If you're buying new tires anyway, it's a good time to ask the shop to check the sensors' age.
Until a bad sensor is replaced, the car drives normally, but the warning system can't warn you. Check your tires with a gauge every couple of weeks.
Why you still need a gauge
A tire pressure sensor is a last warning, not a full check. A tire can be 15 or 20% low, costing you gas and wearing unevenly, without turning on the light. That's why tire makers and safety groups still say to check your pressure once a month and before long trips, especially when the weather turns cold: tires lose about 1 PSI for every 10°F drop. Our cold weather tire pressure calculator shows how much yours have likely lost.
You don't have to pay for a check. 2,674 tire shops on our map belong to chains that say every store will check your tire pressure for free, including Discount Tire, Les Schwab and Goodyear Auto Service, and many more will do it if you ask. Search the free air map and turn on the Pressure check filter to see the ones near you. For a step-by-step walkthrough, read how to check tire pressure, or keep our free tire check card in your glove box.
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What does a tire pressure sensor do?
It watches your tire pressure and turns on the dashboard warning light when a tire is about 25% below the pressure on your door sticker. Most cars have a small sensor inside each wheel that radios the pressure to the car. Others have no sensors in the wheels and instead notice when a low tire starts spinning faster than the others.
What does TPMS mean?
TPMS stands for tire pressure monitoring system: the sensors and warning light that tell you when a tire is low. The TPMS light is the yellow symbol shaped like a horseshoe with an exclamation point.
Where is the tire pressure sensor located?
On most cars, inside each wheel, attached to the inside end of the valve stem where you add air. You can’t see it without taking the tire off the wheel. Cars with an indirect system have no sensors in the wheels at all.
How long do tire pressure sensors last?
Sensors inside the wheels run on sealed batteries that usually last around 5 to 10 years. When a battery dies, the whole sensor is replaced, and the tire pressure light starts blinking when you start the car.
Can I drive with a bad tire pressure sensor?
Yes, the car drives normally. But the warning system can't tell you if a tire goes low, so check all four tires with a gauge every couple of weeks until the sensor is replaced.
Why doesn't my car's tire pressure reading match my gauge?
A difference of 1 or 2 PSI is normal. Sensors read the tire as it warms up while you drive, and gauges vary a little. Set your pressure with a gauge when the tires are cold, using the number on the driver's door sticker.