Tire pressure monitors, required on all cars and light trucks since 2008, come in two types: direct and indirect. The direct system is more sophisticated and uses sensors mounted to each wheel or tire valve that monitor tire pressure and send the information to a computer that translates the data into a dashboard display. The display will either highlight which wheel is lower on air than recommended or display the actual pounds per square inch of pressure for each tire while the vehicle is moving.
The indirect system uses a vehicle’s antilock braking system to detect when one tire is at least 25 percent lower on pressure than the others based on differences in rotational speeds. Indirect systems display a yellow warning light on the dashboard, but they do not indicate which tire (or tires) is low. The driver must check them the old-fashioned way – with a tire pressure gauge.
With either system, federal regulations don’t require a dashboard warning until one tire is at least 25 percent underinflated, which tire experts say is too late because it reduces the load-carrying capability of the tire, increasing the risk of tire failure.
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