According to the regulations, all light duty vehicles built after September 1, 2007 that fall under the regulations
must be equipped with a system. Two types of monitoring systems are allowed.

Indirect systems use the anti lock brake system wheel speed sensors to monitor differences in tire rotational speeds.
A tire with low pressure has a smaller rolling radius and will rotate faster. Indirect systems can't monitor actual tire pressure.
They only compare tire rotating speeds.

Direct systems are the most common system installed on new vehicles. These systems have a pressure sensor/transmitter
attached to each of the vehicle's wheels, where they sense actual tire pressure. This information is transmitted by radio
frequency to a receiver inside the vehicle, which monitors the system and activates warnings.

On many vehicles, it is even possible to read tire pressures on driver information displays so you don't have to leave your
vehicle to check tire pressures.

Tire pressure sensors on direct systems may be strapped to the wheel centre or they may be part of the valve stem assembly.

If you see a large metal valve stem that is held in place by a nut (who is not your husband), then you probably have a system
on your vehicle, but some of the new systems are using a replaceable rubber valve stem that looks very much like a
conventional valve stem. Many sensors are powered by a small battery that will last about 10 years, maybe.

Some sensors are powered by an internal crystal that excites itself due to road shock.

All these direct systems work equally well but please tell us if you have one on your vehicle before you have any
tires changed or repaired, so we can notify the repair technician.

It is easy to damage a sensor when removing a tire, especially if the person is not aware it has a sensor on the wheel.

There are several manufacturers of original equipment and aftermarket tire pressure monitoring systems.

Beru systems are used by Audi/Volkswagen, BMW, Land Rover, Mercedes-Benz and Porsche.
Schrader make systems for Ford, Chrysler, GM and Nissan/Infiniti.
Lexus has used a system by Pacific.
Siemens is another provider of monitoring systems.

As usual, to make life difficult and as you would expect, even though several auto manufacturers may use systems
from the same supplier, there are few similarities between manufacturers.

Valve stem attachments are usually different. The shape of the sensor differs so it will fit different wheels.
The only thing all systems with valve stem sensors have in common is that they use a special nickel-plated valve core.
Install a regular brass valve core in one of these sensors and it will corrode and seize in the sensor.

Then you will have to replace the complete sensor the next time you want to add air.

Tire pressure monitoring systems may seem like a good idea.

However, they only warn when a tire is getting dangerously low.

These warning systems do not replace regularly checking the tire pressures.

There are still some problems with the systems too.

Living in a cold climate, means there is a considerable change in tire pressure between a reasonably warm garage
and a frigid winter morning.

Many of the vehicles will warn of low pressure because the drop in temperature lowers tire pressure.

This can happen every morning, even though the tire pressures are adjusted correctly.

The systems may not be perfect, but they do direct you to check the tires before tire pressure becomes critically low.