Tire Pressure Light On? What Your Tire Air Pressure Sensor Is Telling You

That yellow light shaped like a horseshoe with an exclamation point in the middle is your tire pressure light. It comes from the tire pressure monitoring system (TPMS), the air pressure sensors that watch your tires for you. Here is how serious it is, why it loves cold mornings, and what to do in the next ten minutes.

How Serious Is It?

A solid light usually means at least one tire is low. US rules require the light to come on when a tire drops 25% or more below the pressure your car maker recommends. That is a real loss of air, so check your tires today.

A light that blinks for about a minute when you start the car, then stays on, usually means the sensor system has a problem, not the tire. The system can't warn you about a low tire until it's fixed, so get it checked at a tire shop soon.

Pull over somewhere safe and call roadside help if a tire looks flat, you heard a bang, the steering pulls hard to one side, the car shakes, or you smell burning rubber. Driving on a flat tire can ruin the tire and the wheel, and it makes the car harder to control.

What the Tire Air Pressure Sensor Does

Most cars have a small sensor inside each wheel, attached to the valve stem (the part you put air in through). Each one measures the air pressure in its tire and sends it to the car by radio. Some cars skip the sensors and instead watch how fast each wheel spins, because a low tire is slightly smaller and turns faster.

Either way, the system's job is to spot a low tire before you can see it. A tire can lose a lot of air before it looks flat.

Why the Light Comes On

These are the usual reasons, most common first:

1. Cold weather. Cold air shrinks. Tires lose about 1 psi (pounds per square inch, the unit on a tire gauge) for every 10 degree Fahrenheit drop in temperature. The first cold snap of fall turns on a lot of these lights. That is real low pressure, not a false alarm, and the fix is often just air.
2. A slow leak. A nail, a leaky valve stem or a bent wheel can let air out over days. If one tire keeps going low after you fill it, this is the likely cause.
3. Normal air loss. Tires lose a little air over time even with no damage. If you haven't checked them in months, they may all be low.
4. A sensor problem. This is the blinking light. Sensors run on a built-in battery that wears out after several years, and a sensor can also get damaged when a tire is changed.
5. New tires, a rotation or the spare. Some cars need the system reset after tire work, and many spare tires have no sensor.

What to Do Right Now

1. Find the right pressure. It is on a sticker inside the driver's door frame (sometimes in the owner's manual). Don't use the number on the side of the tire. That is the most the tire can hold, not what your car needs.
2. Check all four tires when they're cold, before you drive far or after the car has sat for a few hours. Driving warms the air and makes the reading look higher.
3. Fill any low tire at a gas station air pump. Many pumps let you set the target pressure and stop on their own.
4. Look at each tire for a nail, a cut or a bulge on the side. A bulge means the tire is damaged inside. Get it replaced before you drive on it much.
5. Give the light a few minutes of driving. On many cars it goes off by itself once the pressure is right.

Light Still On After You Fill the Tires?

If the pressure is right on all four tires and the light stays solid, check the spare if it has a sensor, then look in your owner's manual. Some cars need you to press a TPMS reset button or choose a reset in the settings menu after you add air.

If the light blinks at startup and then stays on, the system has a fault. Common causes are a sensor with a dead battery, a sensor damaged during a tire change, or a new set of wheels without sensors. A tire shop can scan the sensors and tell you which one isn't reporting.

What a Shop Will Check (and What to Ask)

A tire shop will usually check the pressure, look for a leak, and scan each sensor with a TPMS tool. If a sensor needs replacing, the new one has to be programmed to your car (a "relearn"), so the price can include more than the part.

Ask your mechanic:

• Which sensor failed, and how did you test it? One failed sensor doesn't always mean all four need replacing.
• Is the relearn included in the quote? Get a written quote that names the part and the labor.
• Is the leak repairable? A nail in the tread can often be patched. A puncture in the sidewall usually means a new tire.

Questions People Ask

Why does my tire pressure light come on when it's cold and go off later? Your tires were low in the cold morning air. As you drive, the air warms up and the pressure rises enough to switch the light off. The tires are still low when cold, so add air.

Is the tire pressure light the same as the check engine light? No. The check engine light is about the engine, fuel and emissions systems. See our guide to the check engine light for that one.

Can I ignore a blinking tire pressure light if my tires look fine? It's not a good idea. While the system has a fault, it can't warn you if a tire starts losing air. Check the pressure with a gauge yourself and get the system checked soon.

Does every car have tire pressure sensors? Cars sold new in the US have been required to have a tire pressure monitoring system for years, but not every system uses sensors in the wheels. Your owner's manual says which kind you have.

Check Your Car's Recalls Free. No Scanner Needed.

A warning light can come from a known defect that a manufacturer already fixes for free. Lewis checks your car against NHTSA safety recalls by year, make and model, with no scanner needed.

Related:

• Check Engine Light On: What It Means and What to Do
• How to Check Car Recalls for Free