A tyre pressure sensor sits inside the wheel and only wakes up when the wheel turns, because the battery has to last ten years. This car's spare, bolted flat in the boot under the carpet, reports every few seconds while the car is parked, with a temperature that rises and a rotation count that climbs at the rate of a wheel at forty.
I am a tyre technician and the sensors are the part of the job that changed everything about ten years ago.
How a sensor works.
Each wheel has a sensor on the valve, inside the tyre. It measures pressure and temperature and it has an accelerometer, so it knows when the wheel is turning. It transmits to the car on a radio frequency, and only when it is moving, because a sensor that talked all day would be flat in a year.
We have a tool that listens to them. Hold it by the wheel and it wakes the sensor and reads it, and it shows the sensor's own log: last transmissions, temperature, and the count of rotations since it was fitted, because the count is how the sensor decides it is moving.
A spare in the boot, if it has a sensor at all, is silent. It is not turning.
One customer's car has a full size spare in the boot with a sensor in it, and the sensor talks.
The car parked on my forecourt, keys on the counter, engine off. The tool by the boot reads the spare transmitting every twenty seconds, which is what a sensor does at speed.
Temperature climbing, the way a tyre warms on a run. And the rotation count going up, at a rate that works out, for a wheel of that size, at about forty miles an hour.
The wheel is bolted flat in the well under the boot floor. I have had the carpet up with the tool in my hand and watched the count climb on a wheel I could put my palm on.
What has been checked.
The sensor. Replaced, with a new one from the box, programmed to the car. The new sensor did it inside an hour.
The car's own display, which shows all four road wheels and, on this model, the spare. The spare shows a pressure, which is correct, and a temperature, which is not: it shows the spare five degrees warmer than the road wheels while the car is parked in the shade.
The accelerometer. The rotation count is the accelerometer's, not the car's, and there is no way to tell a sensor it is turning except to turn it.
Radio. Another car's sensor with the same ID, parked nearby, would confuse the tool and the car, and I have checked the ID and there is one of it, and it is in that boot.
The temperature.
A tyre on the road warms with friction and stops warming when the car stops. The spare's temperature climbs while the car is parked, over about twenty minutes, to what a tyre at forty on a warm road reads, and then holds, and I have put a thermometer on the tyre in the boot at that moment and it is cold.
The sensor reads the tyre it is in. The tyre it is in is cold. The sensor reports a warm tyre at forty, and its rotation count says it has done, since it was fitted, a distance roughly equal to what the car's odometer has done in the same time.
The spare has driven every mile the car has, flat in the boot, on its own.
What is done about it.
Nothing needs doing. The spare is inflated, the car's warning light is off because the sensor reports a good pressure, and the customer has never had a puncture and has never had the spare out.
I told her it was a sensor with a mind of its own. She said her husband had always said the same about the car, and that he had been the one who chose it, and that he had been gone two years.
What I do.
I fit tyres and I read sensors.
The one thing I have changed is that when that car comes in I read the road wheels and I leave the spare alone.
The reason is the day I took it out. I lifted the spare out of the well to check the sensor's valve, because that is what you do with an odd reading, and I set it on the floor of the workshop on its side.
It rolled. Not off a slope. It stood itself up off its side on a flat concrete floor, which a wheel does not do, and it rolled about four metres across the workshop at a walking pace in a straight line, and stopped, and the tool in my hand showed the rotation count going up by the number of turns it had taken.