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[Tideland Prius]

Direct Tyre Pressure monitoring system means the sensor is in the valve of each tyres. This is the more expensive version of Toyota's tyre pressure monitoring systems.

Here's an explanation from PriusChat:

"The new system, which is on the 06 prius, allows for user-defined tire pressures, which for us and our slightly more inflated tires for better mileage, is great. hence the reset button for the direct sensors. you can set your own tire pressure and instead of waiting for the pressure to drop 25% below the specified pressure (which would have been whatever is in the door sill, i think it's 35/33) rather than getting a warning light when the pressure dropped to 25% below your preferred 42/40.

still the same principle aside from that. sensors integrated with the valvestem that measure pressure in your tires and transmit the info to a TPMS antenna, which is wired into the TPMS ECU. it sends a warning light on the dash when you lost 25% of your tire pressure.

sensors are run on Li-ion battery that lasts about 10 years and are in sleep mode until they are installed to prevent battery rundown while sitting in your local parts department."

So there you have it. The reset button allows you to set the pressure at which the system will compare the pressure to and 25% below that, the warning light comes on.