Peyton M., Farmington, Conn.

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. Tire_pressure_monitor
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.

Learn more

Answered by Rick Popely on March 17, 2010 in Glossary , How Does That Work? | Permalink

Search Results Search Results for


See if your question has already been asked and answered

Thank You!

Your question has been successfully submitted to It will now be reviewed by our editors and we'll answer it soon if we think it's a useful question. You will be notified via e-mail when the answer is posted. tackles your questions about new cars and the car-buying process. Unfortunately we can't answer questions regarding:

  • Used cars.
  • Most aftermarket products.
  • Mechanical issues. You can visit our friends at Car Talk to discuss your mechanical problems.
Thanks for your interest.


Have our experts answer any of your questions about new cars.

Email us at

Maintenance Advice
Get answers from the
Car Talk Community