Why Your Tire Pressure Light Comes On When It's Cold (and What to Do)

It's the first cold morning of the year. You start the car, and there it is: the amber tire-pressure light. Nothing hit the tires overnight and there's no nail in sight. So what happened?

In most cases, nothing is wrong with your tires. Cold air simply takes up less space than warm air, so the pressure inside the tire drops when the temperature does.

How much pressure do tires lose in the cold?

A common rule of thumb is that tire pressure drops by about 1 PSI for every 10°F the temperature falls. If your tires were set to 35 PSI on a 70°F afternoon and the temperature drops to 30°F overnight, they could read around 31 PSI in the morning. That's enough to trigger the warning light on many cars.

Tires also lose a little air naturally over time, even without a leak. Combine that slow loss with a cold snap, and the light often comes on the first frosty week of the season.

When does the tire pressure light come on?

The light comes from your car's tire pressure monitoring system (TPMS). In the US, these systems are designed to warn you when a tire is significantly under-inflated, roughly 25% below the recommended pressure. In other words, by the time the light is on, the tire really does need air.

• Solid light: one or more tires are low. Check and top them up.
• Flashing light, then solid: usually a problem with the TPMS itself, such as a sensor. Have it checked.

How to check tire pressure the right way

1. Find your number. The correct pressure for your car is on the sticker inside the driver's door frame, or in the owner's manual. The number on the tire's sidewall is the maximum the tire can hold, not what your car needs.
2. Check when the tires are cold. Driving warms the tires and raises the reading. The most accurate time is in the morning, before you drive, or after the car has been parked for a few hours.
3. Check all four (and the spare, if you have one). One low tire is easy to miss by eye.
4. Top up to the door-sticker number, not more. If you check after a short drive, don't let air out to reach the number; the reading will settle when the tire cools.

Should I add extra air in winter?

Set your tires to the number on the door sticker when they're cold. That number already accounts for normal driving. If it's going to be much colder tomorrow than today, it's fine to check again then. What you want to avoid is driving for weeks on tires that are well below the recommended pressure, because under-inflated tires wear faster, can hurt fuel economy and make the car handle worse.

What if the light keeps coming back?

If you top up a tire and it's low again within a day or two, it's probably not just the cold. Look for a nail or screw in the tread, check the valve stem, and have the tire inspected. A portable inflator can get you to a shop, but it can't fix a puncture.

Topping up at home instead of at the gas station

The easiest way to stay on top of cold-weather pressure drops is to check your tires where you park. A cordless inflator like BARIO ProPump lets you screw the hose onto the valve, set the number from your door sticker and press start. It stops on its own when it reaches that number.

A few honest notes: ProPump is made for topping up, not for long, non-stop jobs. It runs for up to 10 to 15 minutes of continuous use per charge, and then it needs to rest and cool down. That's plenty for topping up four car tires that lost a few PSI overnight, but a completely flat tire takes much longer.

Want a reminder when the cold season starts? Our tire pressure guide has the basics in one place.

Quick checklist for cold mornings

• Light on after a cold night? Check all four tires before driving far.
• Use the door-sticker number, not the sidewall number.
• Check cold, top up to the recommended pressure.
• Light back on within a day or two? Look for a leak.