Indirect TPMS vs. Sensor TPMS

Indirect TPMS
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My 2015.5 XC70 T6 (bought used) came with the "Tire Monitor" system (indirect TPMS) -- this flavor of TPMS does not have a pressure-sensor in the wheel. Rather, it uses the wheel speed sensors,

looking for faster rotation of the wheel. Tire circumference shrinks slightly with loss of pressure, therefore has to revolve faster to go the same distance. Volvo claims the indirect system can spot a single tire going soft. But, if two tires are going soft, it reports a "low tire pressure" non-specific to tire location (all 4 tires go yellow in the graphic). When you top-off pressure after a warning comes on, you have to go into MyCar / Tire Pressure, and ask to Recalibrate (only enabled when the car is parked). You have to drive a short distance for the recalibration to complete.

These features lead to some speculation on my part as to how the software detects a lone low-pressure tire. However, I'd rather have someone comment who has tested the indirect system extensively with a calibrated digital pressure gauge, or has insider info on the software algorithm.

Sensor-based TPMS
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This system has a pressure sensor module (battery-powered) in the filler stem of each wheel. Each sensor has a unique ID code, and radio-broadcasts a pairing of ID + pressure reading, picked up by a common TPMS receiver. When first installed, each ID must be associated with its 1 of 4 tire locations -- done at the factory. The battery is known to last about 10 years (shorter based on climate?). There is no way to replace just the battery. When the TPMS modules are replaced with a set of new tires, there is a recoding step. Dealers can do this recoding. Tire center consultants "will try" to do this, but no guarantees.

A set of 4 new TPMS modules/stems can be purchased as a set cheaper than buying 4 individually -- the set is p/n 31414189, about $250 (VolvoWholesaleOEMParts.com).