[Fanis]

The system works by calculating the difference in rotation speed of each wheel (so if you have a flat tyre the diameter of the wheel is smaller and thus it spins at higher rpms) and it does not actually read the tyre pressure. So, it needs a significant drop in tyre pressure for the diameter to be altered and the system to read it and register it. In my car I have found it needs to drop below 20psi to give me a warning.

I read that BMW is launchin a new and improved system from March 2014 (in all models bar the X6) onwards.