AdvertisementHB Auto & AC logo Car trouble is stressful. Friendly help for all makes, European & A/C · Huntington Beach · Mon–Fri (714) 848-1224 Talk it through
AutoRepairAnswersCalm, sourced car-repair answers
Ask a question

Answer

Why Is My Tire Pressure Light On?

Educational guide · sources linked inline · reviewed · 6 min read

Quick answer

The short answer: when the light is on and steady, at least one tire is significantly underinflated, and you should check all four (plus the spare) with a gauge as soon as you can. [NHTSA]

The tire pressure warning, the horseshoe-shaped symbol with an exclamation point (some cars show a top view of the car with all four tires instead), is one of the most common lights drivers ask about. It is easy to ignore because the car usually still drives normally. This answer explains what the light is actually telling you, why its behavior changes, how to check and set your pressures the right way, and why the light should never be your only tire check.

The short answer: when the light is on and steady, at least one tire is significantly underinflated, and you should check all four (plus the spare) with a gauge as soon as you can. [NHTSA]

What this means for you

Seeing a warning light pop up can be unsettling, but a steady tire pressure light is usually a request to check your tires, not a sign the car is about to fail. Most of the time the fix is as simple as adding air at the right pressure. The one pattern that needs a technician is a light that flashes and then stays on, because that means the system itself needs attention.

What the TPMS light is and which cars have it

TPMS stands for Tire Pressure Monitoring System. All passenger cars, light trucks and vans from model year 2008 onward are required to have one. Some 2006 and 2007 models have it too, so owners of those years should check the owner’s manual. [NHTSA]

There are two basic designs. A direct system has a pressure sensor inside each wheel. An indirect system has no pressure sensors at all. It uses the wheel-speed sensors and other vehicle data to work out when a tire has lost air. Either way, an onboard processor reads the signals and turns on the dashboard symbol when pressure falls below an acceptable level. [NHTSA]

Federal rules allow two warning symbols: the tire cross-section with an exclamation mark, and the top view of a car with its four tires. Both mean the same thing. [NHTSA]

Solid, flashing, or on-and-off: reading the pattern

Steady light

A steady light means at least one tire is significantly underinflated. Inspect the tires and check pressures as soon as possible. The light should go out once the tires are properly inflated. [NHTSA]

Comes on in the morning, goes off later

This pattern is common on cool mornings. NHTSA explains that the pressure is likely marginally low: it dips below the warning threshold overnight, then rises to an acceptable level as the tires heat up from driving or the day warms. It still means you should check and correct the pressure. A properly inflated set of tires should not trigger the light. [NHTSA] Tire pressure drops whenever outside temperatures drop, which is why the light often shows up when the weather turns. [NHTSA winter tips]

Flashing for about a minute, then steady

This one is different. On 2008 and newer vehicles, the TPMS must detect and report its own malfunctions. A light that flashes for 60 to 90 seconds and then stays on signals a system fault, such as a sensor problem, rather than simply a low tire. The sequence repeats at every start until it is repaired. Adding air will not clear it. [NHTSA]

How to check and set tire pressure correctly

The most common mistake is using the wrong number. The correct pressure is the one the vehicle manufacturer lists on the Tire and Loading Information label, usually on the driver’s door edge or the B-pillar, or in the owner’s manual. The pressure molded into the tire sidewall is the most that tire can hold. It is not the right pressure for your car. [NHTSA] [FuelEconomy.gov]

  1. Check cold. “Cold” means the car hasn’t been driven for at least three hours. Driving warms the tires and raises the reading. [NHTSA]
  2. Check all of them, including the spare. [NHTSA]
  3. Too high? Release air slowly by pressing the valve core with the edge of your gauge until you reach the correct pressure. [NHTSA]
  4. Too low? Note how many psi are missing and add exactly that amount. [NHTSA]
  5. Recheck that every tire matches the label. Some vehicles specify different front and rear pressures. [NHTSA]

If you have to fill up while the tires are warm, NHTSA says to inflate to the recommended cold pressure anyway. Running slightly under the spec is safer than driving on a badly underinflated tire. Recheck when the tires are cold. [NHTSA] Keep a gauge in the car, because a pothole or a curb can make a tire lose pressure suddenly. [NHTSA]

Why the light is not enough

A TPMS warns only when a tire is significantly underinflated. NHTSA is explicit that it is not a substitute for monthly gauge checks. Underinflation is also hard to see, so looking at a tire won’t tell you either. [NHTSA] Tires lose some air naturally over time even without a puncture, and NHTSA calls underinflation the leading cause of tire failure. [NHTSA summer tips]

The habit pays off. NHTSA reports that only 19 percent of consumers properly inflate their tires, and that proper inflation can extend average tire life by about 4,700 miles. [NHTSA] FuelEconomy.gov puts the mileage benefit at 0.6% on average and up to 3%, with mileage falling about 0.2% for every 1 psi drop in average pressure. [FuelEconomy.gov]

When the light means a shop visit

  • The light flashes and then stays on (system malfunction). [NHTSA]
  • One tire keeps losing pressure after you fill it. That suggests a slow leak, valve or bead problem.
  • You see cuts, cracks, bulges or uneven wear. NHTSA says to stop using a damaged tire and consult a tire professional. [NHTSA]
  • The tires are near 2/32 inch of tread, the point at which NHTSA says they are no longer safe. [NHTSA]

Bring notes: which tire, how fast it loses air, and whether the light is steady or flashing. They help a technician find the cause quickly.

Calm next steps

  1. Pull over somewhere safe and comfortable when it’s convenient. You don’t need to stop in traffic for a steady light.
  2. Look at all four tires for anything obviously flat or damaged.
  3. At the next gas station, check each tire with a gauge and fill to the door-label number.
  4. If one tire keeps dropping, or the light flashes, book a tire check. It’s a routine visit.

Frequently asked questions

Should I inflate to the number on the tire sidewall?

No. The sidewall number is the tire’s maximum pressure. Use the vehicle maker’s cold pressure on the door-edge label or in the owner’s manual. [NHTSA]

What does a flashing TPMS light mean?

On 2008 and newer vehicles, a light that flashes for 60 to 90 seconds and then stays on means the system itself has a malfunction. [NHTSA]

Why does the light come on only on cold mornings?

Pressure is probably marginally low. It dips below the warning threshold overnight and rises again as the tires warm up. Check the pressure with a gauge. [NHTSA]

Sources

  1. https://www.nhtsa.gov/equipment/tires
  2. https://www.nhtsa.gov/winter-driving-tips
  3. https://www.nhtsa.gov/summer-driving-tips
  4. https://www.fueleconomy.gov/feg/maintain.jsp
  5. https://scripps.ucsd.edu/news/voyager-can-may-gray-be-predicted
  6. https://www.weather.gov/media/wrh/online_publications/TMs/TM-277.pdf

General education, not a diagnosis. See our disclaimer.

Questions? (714) 848-1224Talk to HB Auto & AC