Tire Pressure Monitoring System (TPMS) Light
Also called: low tire pressure light, TPMS light, the "horseshoe with an exclamation point", flat tire monitor, tire pressure warning
- amber lamp
A flat-bottomed horseshoe or U shape — a cross-section of a tire, often with tread marks along the bottom — with an exclamation point in the middle. Some vehicles use the text TPMS, or a top-down car outline with the affected tire highlighted alongside a pressure readout.
Check all four tires with a gauge, including the ones that look fine, and set them to the door jamb placard pressure while the tires are cold. Do not judge by eye — a modern radial can be 10 psi low and look completely normal. If one tire reads much lower than the other three, you have a leak rather than a cold morning: look for a nail or screw, listen at the valve stem, spray soapy water around the bead, and get it inspected instead of topping it up every few days. If the steering pulls, the car feels vague, or you hear rhythmic thumping, slow down and get somewhere safe now; a tire losing air quickly at speed is a different problem from a light on a cold morning. After inflating, drive a few miles and the light will normally go out on its own.
- Can I drive
- After you have checked the pressures with a gauge and corrected them, yes. Before you check them, you genuinely do not know what you are driving on, which is the whole point of the light. A tire 25 percent low is a "drive to the nearest air pump" situation, not a "drive across the state" one: underinflation builds sidewall heat, lengthens stopping distances, dulls steering response, and destroys the tire from the inside. A tire below roughly half its placard pressure, one that is visibly bulging at the sidewall, or one that keeps going down gets the spare or a tow, not another top-up. If the light is flashing — a system fault — and all four pressures check out correctly by gauge, the car itself is fine to drive; you simply have no monitoring until it is repaired, so check by hand more often.
Steady: at least one tire has fallen to or below the pressure the system watches for. On US vehicles built since September 2007 that threshold is 25 percent below the placard pressure, which is already low enough to hurt braking, handling and tire life — the light is late, not early. Flashing for about a minute at startup and then going solid means something different: the monitoring system itself has a fault and is not watching your tires at all.
The light tells you a threshold was crossed. It does not tell you which tire, how low it is, or why — unless your vehicle has a per-tire pressure display, and even then the displayed value can lag reality by a few minutes of driving. Correct pressure is the number on the placard in the driver's door jamb, measured cold. It is not the number molded into the tire sidewall; that is the maximum pressure at maximum load and is not a recommendation. Cold means before driving, or after the car has sat several hours. Tires gain a few psi as they heat up, so pressure set at a gas station after a highway run leaves you underinflated the next morning. Temperature is the reason most of these lights appear. Pressure changes roughly 1 psi for every 10°F of ambient change, so the first cold snap of autumn lights up an enormous number of cars that were perfectly fine a week earlier. There are two kinds of system. Direct TPMS puts a battery-powered radio sensor inside each wheel, usually built into the valve stem; the batteries are sealed, last several years, and when one dies the whole sensor is replaced. Indirect TPMS has no sensors at all — it infers a low tire from wheel speed data, because an underinflated tire has a smaller rolling radius and turns faster. Indirect systems must be reset manually after inflating or rotating, and they are blind to all four tires going down together. One more thing the light cannot tell you: whether the problem is solved. A tire refilled once after a nail will come back. The light going out means the pressure came up, not that the tire is sound.
This is the distinction most drivers miss. A TPMS lamp that flashes for roughly a minute at every startup and then stays on solid is reporting a fault in the monitoring system — a sensor not transmitting, a dead sensor battery, a missing or unprogrammed sensor, or a receiver problem. It is not telling you a tire is low, and, crucially, it will not tell you if one goes low. Check pressures by hand and have the system repaired. The exact flash duration varies slightly between manufacturers, but the flash-then-solid pattern is close to universal on US-market vehicles.
- A drop in ambient temperature. Roughly 1 psi lost per 10°F, so the first genuinely cold morning of the season triggers cars that were fine in warm weather.
- A slow leak — a nail or screw in the tread, a leaking valve core, or a bead leak where a corroded alloy wheel meets the tire.
- Corroded aluminum valve stems on direct sensors, and the rubber seals and grommets that should be replaced any time a tire is dismounted.
- A dead sensor battery. The batteries are sealed inside and last several years; when one dies the sensor is replaced as a unit.
- A sensor never relearned to the vehicle after new tires, a rotation, or a wheel swap — the car is listening for an ID that is no longer where it expects it.
- Winter wheels, aftermarket wheels, or a full-size spare fitted without sensors, or with sensors that were never programmed.
- A sensor physically broken during tire service, which is common enough that a light appearing the day after new tires is usually about the tire shop, not the tire.
- Receiver module, antenna or wiring faults, or radio interference. On indirect systems, the same wheel speed sensor problems that upset ABS.
- Ordinary permeation. All tires lose a little air every month, so a car that has not seen a gauge in a year is often just low.
A warning light reports that a module detected something. It does not identify which part is responsible — that takes reading the stored codes and testing.
Inflate to the door jamb placard pressure while the tires are cold, then drive. Most systems put the light out within a few minutes above roughly 15-20 mph. If it stays on afterward, either a tire is still below threshold, a sensor is dead or unlearned, or the system has a fault. Indirect systems need a reset through the dash menu or a reset button after inflating — they take your current pressures as the new baseline, so never reset one before setting the pressures correctly. Relearn procedures for direct systems vary widely: some learn automatically after a few miles of driving, some need a TPMS activation tool held at each wheel in a set order, some are done through the OBD-II port, and some older ones use a magnet or an ignition and pedal sequence. The procedure is vehicle-specific — look it up for your exact model rather than assuming. Rough US independent-shop costs: sensor $50-$150 each in parts, and about $75-$200 per wheel installed once dismounting, a new service kit, remounting and balancing are included. Relearn or programming $25-$75. A service kit — valve core, grommet, cap — is a few dollars and should be replaced at every tire change; skipping it is why stems leak. A proper patch-plug repair from inside the tire runs $30-$60. An outside-only plug is a roadside measure, not a repair, and punctures in the sidewall or shoulder are not repairable at all.
The light tells you a system reported a fault. The stored code tells you what it measured. Look up your code — or if you have several, analyse them together.