Why the Light Comes On When It Gets Cold
It's one of the most common dash lights of the season. The weather turns, the tire pressure warning comes on overnight, none of the tires look flat, and a few miles later the light goes out on its own. On cars with a pressure sensor inside each wheel, BMWs among them, it can seem to happen the very first morning the temperature drops.
The reason is physics. Air contracts as it cools, and AAA puts it plainly: tire pressure decreases about 1 PSI for every 10°F drop in temperature. Redding's winter days are mild, but the nights aren't. NOAA's climate normals for the Redding airport show January averaging a high of 57.2°F and a low of 37.7°F. That's a swing of roughly 20 degrees between afternoon and morning on an average day, or about 2 PSI by AAA's rule of thumb, and more on a clear, cold night.
Tires also lose a little air on their own over time. If yours were last set on a hot afternoon back in summer, they've been sliding lower all fall.
Why It Goes Off After a Few Miles
The tire pressure monitor isn't telling you a tire is at the recommended pressure. It's telling you a tire has dropped well below it. Under the federal standard, NHTSA's FMVSS No. 138, the warning is designed to come on when a tire is about 25% below the recommended cold pressure, and AAA notes the light typically doesn't come on until a tire reaches that point.
So on a cold morning, tires that were already a bit low cross the line. Drive a few miles, the tires warm up, the pressure climbs back above the threshold, and the light goes out. Nothing got fixed. The tires are still low, and they'll be lower again tomorrow morning. Treat the light going out as a reminder, not an all-clear.
Running low isn't harmless. An underinflated tire flexes more, builds more heat, wears its outer edges faster, and doesn't grip or handle the way it should, which matters most on wet winter roads.
How to Set Your Tire Pressure Correctly
- Check them cold. AAA defines cold as the vehicle not having been driven for at least three hours. First thing in the morning, before you leave, is ideal.
- Use the number on the door-jamb sticker. The recommended pressure for your vehicle is on the placard on the driver's-side door jamb. The number molded into the sidewall is the tire's maximum, not what your vehicle needs.
- Add air to that number. Use a decent gauge of your own rather than trusting the readout on a gas station hose.
- Check at least once a month and before a long trip, as AAA recommends. In fall and winter, expect to add air more than once as the nights get colder.
- Never let air out of hot tires. After driving, tires read higher than they will once they cool. Bleed them down to the sticker number while they're hot and they'll be low by morning.
Should You Inflate for Winter and Deflate for Summer?
This is the question people actually ask, and the answer is no. There's no separate winter pressure and summer pressure. There's one number, the one on the door-jamb sticker, and the goal is to hit it cold, all year.
What changes with the season is what it takes to stay at that number:
- Fall and winter: the temperature keeps dropping, so you'll be adding air to get back up to the placard pressure.
- Summer: check cold, first thing in the morning. If they read at the placard number cold, leave them alone, even though they'll read higher on a hot afternoon. That rise is normal.
Deliberately overinflating for winter or underinflating for summer doesn't help in either season.
Some Cars Need You to Confirm or Reset It
Some vehicles, many BMWs among them, need the tire pressure monitor confirmed or reset in the settings menu after you adjust pressures. Your owner's manual shows how. On some of these systems the reset stores whatever pressures are in the tires at that moment, so set all four correctly, cold, before you reset, not after.
There are two broad types of system. Direct systems have a battery-powered sensor inside each wheel that reports actual pressure. Indirect systems have no sensors in the wheels; they use the anti-lock brake wheel speed sensors to notice a tire turning faster than the others, which is what a low tire does. Either way, the fix for a cold-weather warning is the same: set the pressures, then follow the manual for the reset if your vehicle needs one.
When It's Not Just the Weather
Cold mornings explain all four tires being a little low together. They don't explain these:
- One tire consistently lower than the others. That's a leak: a nail or screw in the tread, a leaking valve stem, or corrosion where the tire seals against an alloy wheel.
- A light that won't clear even after every tire is set to the sticker pressure and the system has been reset.
- A flashing light. On many vehicles, a tire pressure light that flashes, or flashes and then stays on, signals a fault in the monitoring system rather than a low tire.
- A warning that started after a tire or wheel change, which can mean a sensor wasn't registered to the vehicle.
Those can mean a failed sensor, a sensor whose internal battery is done, or a system fault, and they're worth having checked.
What We Check
- Every tire's pressure, cold, with an accurate gauge, including the spare if it has a sensor.
- Each tire for punctures, valve stem leaks and bead leaks, using soapy water where needed.
- Each sensor, read with a TPMS scan tool: whether it's reporting, what pressure it sees, and on many sensors, whether its battery is low.
- The system for stored fault codes, plus a relearn or sensor programming when a sensor is replaced or a wheel has moved.
TPMS sensor service and programming are part of our tire services, and a warning that points to a deeper fault gets a proper look through our vehicle diagnostics. You get a written estimate before any repair, and the work is backed by our 12-month/12,000-mile warranty on parts and labor.
Nights up the hill are colder still. If you're driving into the mountains this winter, read winter car prep for Redding drivers too.
Tire pressure light that won't stay off? Call (530) 785-9900. 5490 Churn Creek Rd, Redding, CA 96002. Mon–Fri 8AM–5PM.