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Tire pressure monitoring systems

Correct tire pressure is critical to safety, and studies show that as many as half the vehicles on the road have improperly inflated tires. That's why an increasing number of modern vehicles include automated tire pressure monitoring systems (TPMS). Federal law requires TPMS on all new cars and light trucks starting from the 2008 model year, but many makers have included it in their vehicles for years.

How does TPMS work, and what do you need to know about it?

Two types of system

These systems can be direct or indirect. Both types alert the driver in three situations: when a tire's air pressure drops 25% below the recommended pressure, when a tire is dangerously overinflated, or when a reading cannot be taken on a tire at all.

Direct systems use an air pressure sensor inside each wheel, which sends a signal to the vehicle's computer every 30 to 60 seconds while the vehicle is in motion. This type of system is very accurate but more costly, as the sensors must be replaced completely if they fail—or after they wear out, which takes five to 10 years.

Indirect systems do not have their own sensors or require installation of any new devices. They simply reprogram the vehicle's wheel speed sensors (part of the antilock braking system) to detect small changes in tire diameter that might come from incorrect inflation. The key word here is "might." Indirect TPMS can give false alarms in slippery road conditions, or when all four tires are not the same size to begin with. Unlike direct systems, which monitor the air pressure of each tire separately, indirect systems compare all four tires and look for any difference from the norm. This means that if two tires—or all of them—are equally low, an indirect system has a much harder time detecting it.

How TPMS affects maintenance

No matter which system your vehicle uses, it's important to ask the technician before any tire service whether he or she is able to reset the monitoring system afterwards. New tires, new wheels or tire rotations will all require a reset. If your technician isn't equipped for it, you could be in for a trip to the dealership and an unexpected additional expense.

Mixing old and new tires can cause problems for an indirect TPMS. If possible, replace all tires at the same time. Check whether the spare tire is being monitored! This varies from one vehicle to another. "False alarms" may actually indicate a problem with the spare.

Finally, if your vehicle uses direct TPMS, be wary of using emergency tire sealants to repair a flat. Sealant can clog the pressure sensors and lead to an expensive replacement.

For more car maintenance tips, visit www.AAA.com.