What a tire pressure monitor does
A tire pressure monitoring system (TPMS) is a sensor in each of your tires that watches the air pressure and sends a signal to your dashboard when pressure drops too low. When the system detects a problem, a warning light appears on your instrument cluster — usually a picture of a tire with an exclamation mark. The light tells you that at least one tire is underinflated, not that you have a flat.
Every car sold in the United States since 2008 has been required to have TPMS. The system does not fix the problem or inflate your tire; it only alerts you that something needs attention. How you respond — whether you check the pressure yourself, visit a gas station, or go to a mechanic — is up to you.
Key Takeaways
- The TPMS warning light means at least one tire is below the manufacturer's recommended pressure, usually by 25% or more.
- Check all four tire pressures with a gauge at a gas station or tire shop, because the system does not tell you which tire is low or by how much.
- Tire pressure changes with temperature — cold weather can trigger the light even if your tires were fine the day before.
- If the 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.
- The correct pressure for your car is printed on a sticker inside the driver's door jamb, not on the tire sidewall.
Direct and indirect TPMS: how they differ
There are two types of tire pressure monitoring systems, and they work in different ways. A direct TPMS has a small sensor and transmitter inside each tire that measures the actual pressure and sends the reading wirelessly to your car's computer. A indirect TPMS does not have sensors in the tires; instead, it uses the car's existing wheel speed sensors (the same ones that run your anti-lock brakes) to detect when a tire is spinning faster than it should, which happens when pressure is low and the tire is smaller in diameter.
Direct systems are more accurate because they measure real pressure. Indirect systems are cheaper to install and have fewer parts to replace, but they cannot tell you the actual pressure — they only know something is wrong. Most cars made after 2010 use direct TPMS. If your car has indirect TPMS and the light comes on, you still need to check all four tires with a pressure gauge to find out which one is low.
Why the warning light turns on
The TPMS light comes on when pressure in one or more tires drops to about 75% of the manufacturer's recommended pressure. If your car calls for 32 pounds per square inch (psi), the light will typically trigger around 24 psi. This threshold exists because underinflated tires wear faster, use more fuel, and can overheat on long drives.
Cold weather is the most common reason the light appears, especially overnight or after a cold night. Air contracts when temperature drops, so tire pressure can fall 1 to 2 psi for every 10-degree drop in outside temperature. If you see the light in winter, check your tire pressure before assuming there is a leak. Warm weather can also affect pressure in the opposite direction — tires heat up during driving and pressure rises, which is why you should check pressure when tires are cold (before driving or at least three hours after driving).
How to respond when the light appears
The first step is to check the pressure in all four tires using a tire gauge. You can do this at most gas stations for free, or buy an inexpensive gauge to keep in your car. Write down the pressure in each tire so you know which one (or ones) is low. Compare the readings to the recommended pressure on the sticker inside your driver's door jamb — not the maximum pressure printed on the tire sidewall, which is a different number.
If one or more tires are low, inflate them to the recommended pressure. Many gas stations have air pumps you can use for free. After inflating, drive for a few minutes and check whether the light goes off. If it does, the problem is solved. If the light stays on after you have inflated all tires to the correct pressure, a sensor may be faulty, a tire may have a slow leak that keeps losing pressure, or the system itself may need service. At that point, a tire shop can test the sensors and check for leaks.
Sensor batteries and replacement costs
Direct TPMS sensors contain small batteries that typically last 5 to 10 years. When a battery dies, the sensor stops transmitting and the warning light comes on even though your tire pressure is fine. Replacing a single sensor usually costs between $50 and $150 per tire, depending on your car and the shop. Some shops charge less if you replace all four at once.
Indirect TPMS systems do not have batteries in the tires, so there is nothing to replace unless the wheel speed sensors (which serve multiple purposes) fail. If you have an indirect system and the light stays on after you have confirmed all tire pressures are correct, the problem is likely with the car's computer or wheel speed sensors, which requires a mechanic to diagnose.
Slow leaks and when to see a mechanic
If the TPMS light comes on and stays on even after you inflate your tires, one tire may have a slow leak. A slow leak can come from a puncture too small to notice, a faulty valve stem, or a crack in the sidewall. To find out, check your tire pressure again after a few days. If pressure has dropped in the same tire, you have a leak.
A tire shop can locate the leak by submerging the tire in water or using a soapy solution to find where air is escaping. Small punctures in the tread can often be patched for $10 to $25. Leaks at the valve stem or sidewall cracks usually mean the tire needs to be replaced. If you ignore a slow leak, the tire will eventually go flat, and driving on a flat tire damages the sidewall and can make the tire unrepairable.
Resetting the light after sensor replacement or repair
After a mechanic replaces a TPMS sensor or repairs a leak, the warning light should turn off on its own once the system detects normal pressure in all tires. This usually happens within a few minutes of driving. If the light stays on, ask the shop whether they reset the system — some cars require the technician to manually clear the warning using a diagnostic tool.
Do not attempt to reset the light yourself by disconnecting the car battery or using other methods you may find online. These methods do not actually fix the underlying problem and the light will return. If the light does not go off after a repair, contact the shop and ask them to check their work.
Frequently Asked Questions
Can I drive with the TPMS light on?
You can drive short distances, but you should check your tire pressure as soon as possible. Underinflated tires wear faster and can overheat on long drives or in hot weather. If a tire is very low or flat, driving on it will damage the sidewall and may make the tire unrepairable.
Why did the light come on when I just checked my tires last week?
Temperature changes are the most common reason. If the weather got colder, tire pressure drops naturally. If you checked pressure when tires were warm, the pressure may have been higher than it actually is when cold. Check pressure again when tires are cold and compare to the door jamb sticker.
Does the TPMS light mean I have a flat tire?
No. The light means pressure is low in at least one tire, but not necessarily flat. A flat tire usually causes the car to pull to one side or feel bumpy. If you see the TPMS light but the car drives normally, check your tire pressure with a gauge — most likely one tire is straightforward underinflated.
What if my car is old and does not have TPMS?
Cars made before 2008 are not required to have TPMS. You can still monitor tire pressure by checking it monthly with a gauge, especially before long drives or when weather changes. Many tire shops will check pressure for free.
Can I ignore the TPMS light if I am only driving to the store?
Short trips are lower risk, but underinflated tires still wear faster and use more fuel. It takes only a few minutes to check pressure and inflate at a gas station, and doing so prevents damage to the tire and keeps your car running efficiently.