Guide

How TPMS sensors work

A direct TPMS sensor is a small, battery-powered radio sealed inside each wheel. It measures tire pressure and temperature, then broadcasts a short packet carrying its own unique ID and those readings on the 315 MHz or 433 MHz band, mostly while the wheels are turning. The vehicle's receiver decodes the packet and warns the driver when pressure drops.

Direct and indirect TPMS are not the same thing

Direct TPMS puts a real pressure sensor in every wheel and transmits measured readings by radio. Indirect TPMS has no sensors at all — it infers an underinflated tire from wheel-speed differences reported by the ABS, because a soft tire has a slightly smaller rolling radius and turns marginally faster.

The practical differences follow from that. Direct systems can tell you which tire is low and what the pressure actually is; indirect systems can only raise a general warning and must be reset by hand after you inflate the tires. Only direct systems put a radio identifier on each wheel, which is what makes a wheel individually recognizable from outside the car.

What a sensor actually transmits

Each burst is short — a fraction of a second — and typically carries a factory-programmed sensor ID, the current pressure, and on most sensors temperature and a status or battery flag. The ID is the important part: it does not change over the life of the sensor.

The vehicle learns its four sensor IDs during a relearn procedure so it knows which corner each reading belongs to. That is why rotating or swapping wheels between vehicles usually requires a relearn — the pressures are still being broadcast, the car just no longer knows whose they are.

When sensors transmit, and when they go quiet

Most sensors transmit on a schedule of roughly every 30 to 60 seconds while the vehicle is moving, then drop into a low-power sleep state when it is parked. Motion is what wakes them, usually detected by an accelerometer or centrifugal switch that trips somewhere in the neighborhood of 15 to 25 mph.

A sudden pressure change can also trigger an unscheduled burst, which is how a car flags a fast leak between scheduled transmissions. The behavior to remember is the quiet one: a car that has not moved in days is broadcasting rarely, if at all.

315 MHz and 433 MHz

Vehicles sold in North America overwhelmingly use 315 MHz, while European and many imported models use 433.92 MHz. Both are short-range unlicensed bands also used by key fobs and garage remotes, which is why a lot of low-cost radio hardware happens to cover them.

Neither band travels far. In open air a TPMS burst carries a modest distance at best, and a yard full of sheet metal and stacked bodies cuts that down further.

Wake-up tools and the 125 kHz signal

A TPMS activation or relearn tool forces a sensor to transmit on demand by sending a 125 kHz low-frequency wake signal from a few inches away. Tire shops use these during relearns; the same trick is useful anywhere you need a parked car's sensors to speak up without driving it.

How long TPMS sensor batteries last

Sensor batteries are sealed inside the housing and are not serviceable, so a dead battery means replacing the whole sensor. Service life is generally quoted in the range of five to ten years, and it depends more on how many transmissions the sensor has made than on calendar age — a high-mileage vehicle wears its sensors out sooner.

Typical TPMS characteristics at a glance

These are the commonly cited, approximate figures. Specifics vary by manufacturer, sensor generation, and model year.

Why any of this matters on a vehicle lot

Because every TPMS-equipped vehicle already carries four radios with unchanging IDs, a lot can be mapped by listening instead of by tagging. That is exactly how Auto Lot Finder works: receivers you mount around the property decode the bursts, and each vehicle's zone updates roughly every 60 seconds with nothing installed in the cars.

The sleep behavior sets the expectations. A car's zone updates within about a minute of the wheels turning, and a vehicle parked for a week simply holds its last known zone, which is still correct. At intake, a 125 kHz activation tool held near each tire — or just driving the car past the intake receiver — makes the binding immediate. Coverage is not universal: roughly 85% of vehicles built since 2008 are recognized out of the box, and unrecognized sensor types are captured automatically so decoders can be added in later updates.