The TREAD act requires that the sensors alert NLT 25% below the set pressure.
Some systems (BMW for one) tend to alert much sooner - but - it alerts not according to the recommended pressure, but at the pressure that the system was reset to.
So, if the dealer initially set the pressure to 42psi (not uncommon) then the system would be required to lert at around 32psi.
In my experience with E90 sensors, you'll get an alert around 20% low.
Remember that the sensors are only intended to alert you to a potentially unsafe tire pressure and is not intended to replace your periodic checks.
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