Have you ever seen a vehicle with one or more tyres that appear noticeably low on tyre pressure? Didn't you want to warn the driver of the situation before that slight inconvenience became a calamity? What if the vehicle with the low tyre pressures is the one you're driving? Wouldn't you want to be warned?

Maintaining the correct tyre pressure for a vehicle is an important factor in how much load its tyres can safely carry. The correct pressure will carry the weight without a problem. Too little tyre pressure will eventually cause catastrophic tyre failure.

Tyres aren't invincible. They are made of individual layers of fabric and steel encased in rubber. If a tyre is allowed to run low on air pressure, the rubber is forced to stretch beyond the elastic limits of the fabric and steel reinforcing cords. When this happens, the bond between the various materials can weaken. If this is allowed to continue, it will eventually break the bonds between the various materials and cause the tyre to fail.

This has made tyre pressure maintenance an important safety issue throughout the automotive industry and caused the U.S. government to pass legislation mandating tyre pressure monitoring systems. The main purpose of these systems is to warn the driver if their tyres are losing air pressure, leaving the tyres under-inflated and dangerous.

There are two main types of tyre pressure monitoring systems in use today. Each works differently and has its own strengths and limitations.

Direct tyre pressure monitoring systems measure, identify and warn the driver of low pressure. Because direct systems have a sensor in each wheel, they generate accurate warnings and can alert the driver instantly if the pressure in any one tyre falls below a predetermined level due to rapid air loss caused by a puncture. In addition, direct tyre pressure monitoring systems can detect gradual air loss over time. Some direct systems use dashboard displays that provide the ability to check current tyre pressures from the driver's seat.

Direct systems attach a pressure sensor/transmitter to the vehicle's wheel inside the tyre's air chamber. Most Original Equipment and some aftermarket systems attach their air pressure sensor/transmitter to special tyre valves. While the presence of a metal clamp-in valve typically identifies the presence of a direct tyre pressure monitoring system, special snap-in rubber valves have also been used to support direct system sensors. The transmitter's signal is broadcast to the in-car receiver and the information is displayed to the driver.

Some aftermarket and Original Equipment direct monitoring systems attach the sensor/transmitter to the wheel with an adjustable metal strap. These sensors/transmitters and their straps only weigh a few ounces and allow virtually universal application on car and light truck wheels.

In the interest of providing a lower cost Original Equipment system, indirect tyre pressure monitoring systems were developed by vehicle manufacturers wishing to comply with the law while minimizing development time and cost. Indirect systems use the vehicle's anti-lock braking system's wheel speed sensors to compare the rotational speed of one tyre to that in another position on the vehicle.

If one tyre is low on pressure, its circumference changes enough to roll at a slightly different number of revolutions per mile than the other three tyres. Reading the same signal used to support ABS systems, the vehicle manufacturers have programmed another function into the vehicle's on-board computer to warn the driver when a single tyre is running at a reduced inflation pressure compared to the others.

Indirect systems (except for the TPMS on several 2009+ Audi models and 2010+ Volkswagen models) are unable to generate accurate readings in cases where all four tyres are losing pressure at the same rate, such as the effects of time and temperature.