[ted2014]

i have a 2018 540xi , one rear run flat tire is showing low pressure . it recommended as 35, after i Pumped up to 35 then after a week, it goes down to 30 and keep this number always, car showing warning sign now. i did it again, then it goes down to 30 again... not sure why, any idea? it just happened recently when cold weather is coming now. not sure if it is normal, but i think once you pumped up your tire, it could last like 1 month at least without doing it again. thanks

[CoolWave911]

Maybe try nitrogen instead of just nornal air. My car always filled with nitrogen and I barely needs to top up ever.

[AP]

Pressure will drop when the temperature drops so you have to keep an eye on these regularly especially when the ambient temperatures change significantly.
Suggestion you can set up a shortcut button to display these for ease of use.

Run flats really need to be at the right pressure for good wear and life , they obviously mask low pressures very well

You might also have a very slow leak in that 1 tyre if it is always going down compared to the others

I personally check my tyres every trip and if the weather forecast predicts that the temperature is due to drop I make an effort to put more air in ahead of that to ensure they are ok .

[hlothery]

This sure sounds like a slow leak to me, like a small nail in the tire. RFs can sometimes be patched like normal tires, but you most likely have to have a shop pull it off to see where the leak is.

[claykin]

Or possibly a leak at the schrader valve. After filling the tire put a bit of soapy water on the end of the valve stem and see if it bubbles.