How it works:

When the car is turned on the warning light on the dashboard will come on for about 2 seconds. This is normal.
If the light flashes (3 seconds off, 1 second on), that indicates a problem with the system. Contact your dealer.

Each sensor has an separate ID code. When the vehicle goes 7-20 mph, a roll switch inside the sensor turns on and the sensor starts to take measurements every 30 seconds, and sends the results about once a minute to a receiver module under the carpet by the drver's door. The sensors are off when there is no motion (or 15 minutes after stopping).

When a low tire (app 26-27psi) is detected a warning light in the dashboard turns on. The warning light turns on, it does not flash.
The light does not indicate which tire is low, so please check all tires.
A blow-out won't show up right away.

When the vehicle stops moving for 15 minutes, the sensor enters 'sleep' or off mode to save battery life.

WHERE? A transmitter with a tire pressure sensor is located inside the each tire's air valve stem. It is designed to detect low pressure only (not high or over-inflated).
The sensor contains a battery. Battery life is 10 years or 100,000 miles. When the battery fails the sensor must be replaced.

top

Inflating and Adjusting tire pressure
Be sure to let the tires cool. After adjusting the pressure, drive the car at least 20 mph (32 km/h) to start the TPMS. If a light was on due to low pressure and the pressure is now normal, the warning light should go off after driving for a few minutes. When a spare or a wheel is replaced without the original pressure sensor being transferred, the low tire pressure warning light will flash. This indicates the TPMS is unable to monitor all four road  wheels. Contact your dealer as soon as possible for sensor replacement or system resetting. Do not inject any aerosol tire sealant in the tires because it will damage the sensor, do not place metal film or any metal parts under the driver's seat because this may effect the signal from the sensors.

cold temperatures? cold weather may cause the air tire pressure to drop so the warning light comes on.
This is normal and tire pressure should be checked and corrected seasonally.
When checking and adjusting tire pressure inside when it is in cold or winter outside, the difference between the indoor and outdoor temperature is large. This means that in cold air, the tire pressure may drop and the warning light come on. To avoid this, when inside, inflate the tire slightly more considering the difference between inside and outside temperatures. Or use nitrogen which is less sensitive to cold or hot outside temperatures.

top
Replace a wheel or use the spare? When you replace the wheel or use the spare tire without the original pressure sensor/transmitter being transferred, the low tire pressure warning light will flash. It needs all 4 sensors feeding it information.

Rotation? The system doesn't know which tire is where so there's no problem with rotations. The dashboard warning light only indicates a low tire but not which tire is low so you have to check all 4.

Parking  Try to make sure the tires aren't pressing hard against the curb when you park.

FLAT repair using a can of pressurized tire repair sealant will clog the sensor and it will have to be replaced

To reset drive the car app 5 miles fater correcting for low pressure.

Tire Pressure Monitor re-set tool: To reset the system you'll need a small handheld computer.
These are available from Subaru at http://subaru.spx.com/index.asp,  or try aftermarket for less money from OTC
Do you know of other systems that will reset the Subaru?

Nitrogen in tires - Some dealers are offering Nitrogen as an alternative to inflating tires with normal outside air. Nitrogen maintains tire pressure better than normal air which leaks out over time, and is less sensitive to cold outside temperatures. As a result, Nitrogen minimizes the Tire Pressure Monitoring light coming on.
And because it maintains tire pressure better and is less sensitive to hot and cold, nitrogen can improve economy, increase tire life, and might be safer because the tires are less likely to blow-out when driven at high speeds for a long distance, or on hot roads.
Nitrogen has been used by Nascar, Formula One, the Tour de France and the military for years.
Mixing air and nitrogen. Don't put normal air in a tire with nitrogen because you will lose the nitrogen benefit, but it's not dangerous.
The downside to Nitrogen is that you might have to pay for it (but air is free).

Watch for this warning light in the instrument panel.  Steady illumination means a tire is low, it flashes if there's a problem with the system.
the sensor and valve stem are i piece.  The sensor goes inside the tire  this design was used in all models until 2007.	valve stem
this tire monitor started on 2008 models.  Notice the brass colored threads	replace valve and stem with every tire change according to schraderservice.com

top

diagram of the wheel &  sensor/air valve stem

the sensor in the wheel is part of the normal air valve stem that  goes through the side        these images © Subaru of America

top