To explain both questions... the tyre pressure monitor works by detecting the suspension levels on all 4 wheels.. if one appears lower, i.e when you get a puncture, it raises the alarm. if all 4 tyres are flat or lose pressure evenly, the sensors wont detect any abnormalities with the suspension as everything is even
i Remember the last time i had a puncture, i had 22psi left in the tyre after the tyre pressure alarm went off.
this is what im led to believe of how it works. im more than happy to be corrected if this isnt the case.
I personaly maintain 36 psi in my tyres regardless of the weather.
Rich