What a tire air pressure monitor system does
A tire air pressure monitoring system (TPMS) is a set of sensors that watch the air pressure in each of your tires and alert you when one drops below a safe level. Every new car sold in the United States since 2007 has one. The system does not fill your tires — it only tells you when pressure is low enough to cause problems.
When your TPMS light comes on (usually a yellow or amber icon shaped like a tire with an exclamation mark), it means at least one tire has lost enough air that the system thinks you should know about it. The threshold varies by vehicle, but most systems trigger around 25% below the tire's recommended pressure. That is not an emergency — you have time to pull over safely and check your tires — but it does mean you need to act within the next few hours or days, depending on how much pressure is lost.
The system runs constantly while you drive. It checks pressure every few seconds and stores the readings. If a tire stays low for a certain amount of time (usually a few minutes of driving), the alert activates on your dashboard.
Key Takeaways
- TPMS sensors in each tire send pressure readings to your car's computer, which compares them to the recommended pressure listed on your driver's door jamb or fuel door.
- A TPMS alert means at least one tire is underinflated, but the system does not tell you which tire or how much pressure it has lost — you must check manually.
- Low pressure can happen from temperature drops, slow leaks, or a puncture, and you should check all four tires with a gauge even if only one alert appears.
- If the TPMS light stays on after you inflate your tires to the correct pressure, a sensor may be faulty or a tire may have a slow leak that keeps returning.
- Replacing a TPMS sensor costs between $150 and $300 per sensor at most shops, but many tire shops can reprogram existing sensors when you move tires around.
How TPMS sensors send pressure data to your car
Each tire has a small sensor mounted inside the wheel, usually attached to the valve stem or the wheel rim. The sensor measures air pressure and temperature and sends that data wirelessly to a receiver in your car, typically located near the engine or under the dashboard. This happens dozens of times per minute while you drive.
The receiver compares the pressure in each tire to the recommended pressure for your vehicle. That recommended pressure is not the maximum pressure printed on the tire sidewall — it is the pressure your car's manufacturer says is correct for your specific vehicle. You will find this number on a sticker inside your driver's door jamb, on the fuel door, or in your owner's manual. It is usually between 28 and 35 pounds per square inch (psi), though some vehicles run higher or lower.
If one tire drops below about 75% of that recommended pressure, the system flags it as low. The exact threshold depends on your vehicle's programming, but the principle is the same: the system is looking for a pressure drop large enough to affect your car's handling, fuel economy, or tire safety.
What happens when your TPMS light turns on
When the system detects low pressure, it sends a signal to your dashboard. Most cars show a warning light, some show a message on the instrument cluster, and a few newer vehicles display which specific tire is low. However, most TPMS systems do not tell you the actual pressure reading — only that one or more tires are below the threshold.
You should pull over at a safe location and check all four tires with a tire pressure gauge. Do not assume only one tire is low just because one alert appeared. A single sensor may be faulty, or multiple tires may be losing pressure at different rates. Check the pressure in all four tires, compare each to the recommended pressure on your door jamb, and inflate any that are low.
After you inflate your tires, the TPMS light should turn off within a few minutes of driving. If it stays on after you have confirmed all tires are at the correct pressure, either a sensor is malfunctioning, one tire has a slow leak that will drop again soon, or the system itself needs recalibration. Take your car to a tire shop or dealership to have the system checked.
Common reasons your TPMS alert appears
Temperature changes are the most common trigger. Air contracts in cold weather and expands in heat. A 10-degree drop in outside temperature can lower tire pressure by 1 to 2 psi. If you park your car overnight in freezing weather, you may see a TPMS alert the next morning even though nothing is wrong with the tire. Inflate to the correct pressure, and the light will go away.
A slow leak is the second most common cause. A nail, screw, or puncture too small to notice may let air out gradually over days or weeks. Check the tire for visible damage, and if you find nothing, take the tire to a shop where they can submerge it in water to find a slow leak. Many slow leaks can be repaired with a plug or patch if caught early.
A faulty sensor is less common but does happen. Sensors have batteries that last 5 to 10 years, and they can fail from corrosion, impact, or age. If your TPMS light stays on after you have confirmed all tires are properly inflated and you have ruled out a slow leak, a sensor has likely failed and needs replacement.
What to do if your TPMS light stays on
First, check all four tires with a manual gauge. Many gas stations have free air pumps, and most tire shops will check your pressure for free. Write down the pressure in each tire and compare it to the recommended pressure on your door jamb. If all tires are within 2 psi of the recommended pressure, the problem is likely a sensor or the system itself.
If one tire is consistently low, mark that tire and monitor it over the next few days. If the pressure drops again, the tire has a slow leak. Take it to a tire shop for inspection. They will clean the tire, submerge it in water, and look for bubbles that indicate a leak. If they find one, they can often repair it for $15 to $30. If they find no leak but the tire keeps losing pressure, the valve stem itself may be leaking and will need replacement.
If all tires hold pressure but the light stays on, a sensor has failed. You will need to replace it. Sensor replacement costs between $150 and $300 per sensor at most tire shops or dealerships, depending on your vehicle and whether the shop has the sensor in stock. Some shops charge less if you bring your own sensor, though this is less common.
TPMS sensor replacement and maintenance
When a TPMS sensor fails, you have two options: replace it with a new one, or replace it with a refurbished sensor. New sensors cost more but come with a warranty. Refurbished sensors are cheaper but may have a shorter lifespan. Most tire shops recommend new sensors, especially if your car is relatively new.
If you rotate your tires or have them replaced, the shop should reprogram your TPMS sensors so they are matched to the correct wheels. This is a straightforward process that takes a few minutes and usually costs $0 to $50. If you skip this step, your TPMS may give false alerts because the sensors are no longer in the wheels the system expects them to be in.
Sensors typically last 5 to 10 years before the battery dies. If your car is older than 10 years and you have never replaced a sensor, you may start seeing TPMS alerts as the original sensors age. This is normal wear, not a sign that something is wrong with your tires.
Understanding TPMS versus manual pressure checks
TPMS is a safety backup, not a replacement for checking your tire pressure yourself. The system only alerts you when pressure is already low enough to be a problem. By the time you see the light, your tires have already lost a significant amount of air. Checking your tire pressure manually once a month — and before long trips — catches problems earlier and helps you maintain better fuel economy and tire life.
To check pressure manually, you need a tire pressure gauge, which costs $5 to $15 at any auto parts store. Check your tires when they are cold (before you have driven more than a mile or two), because driving heats the air inside the tire and gives a false high reading. Compare each tire's pressure to the recommended pressure on your door jamb, not the maximum pressure on the tire sidewall. If any tire is more than 2 psi below the recommendation, inflate it.
Many newer cars have a digital TPMS display that shows the actual pressure in each tire on the instrument cluster or infotainment screen. If your car has this feature, you can check pressure without getting out of the car. However, this display is still not a substitute for a manual gauge, because the readings may lag by a few minutes and the system may not update if you are parked.
Frequently Asked Questions
Can I drive with my TPMS light on?
You can drive short distances to a tire shop or gas station to check and inflate your tires, but you should not ignore the alert or drive long distances with underinflated tires. Low pressure causes uneven wear, reduces fuel economy, and can overheat the tire, which may lead to a blowout. Check your tires as soon as you safely can.
Why does my TPMS light come on in winter but go away in summer?
Cold air contracts, lowering tire pressure. A 10-degree temperature drop can reduce pressure by 1 to 2 psi. If your tires are already close to the minimum threshold, a cold night may push them below it. Inflate your tires in winter and the light should turn off. This is normal and does not mean your tires are damaged.
What if only one tire shows low pressure?
Check all four tires with a gauge, not just the one you think is low. A single sensor may be faulty, or multiple tires may be losing pressure at different rates. If only one tire is actually low and the others are fine, that tire likely has a slow leak or a faulty valve stem and should be inspected by a tire shop.
Do I need to replace my TPMS sensors when I buy new tires?
Not usually. TPMS sensors are mounted inside the wheels, not on the tires themselves. When you buy new tires, the shop will move your old sensors to the new wheels (or new wheels if you are buying those too). However, if a sensor is old or faulty, the shop may recommend replacing it at the same time.
How much does it cost to fix a TPMS problem?
If the problem is low pressure from a slow leak, a tire repair costs $15 to $30. If a valve stem is leaking, replacement costs $20 to $50. If a sensor has failed, replacement costs $150 to $300 per sensor. A manual pressure check at a tire shop is usually free, so start there to figure out what the actual problem is before paying for repairs.