I have a 2001 525i with 73K miles which I bought new. The computer says I'm almost due for new microfilters. My first set lasted 50K miles and my driving style hasn't changed since my originl microfilters were replaced. Does anyone have any insight as to why the the computer thinks that the second set needs to be replaced twice as quickly as the first set? Thanks in advance.

Even BMW can't work this issue out, which is why they removed the Microfilter CBS from the newer car fitted with the CIC. The microfilter is now a fixed interval replaced item.

The way I understand it, the micro filter is a fixed interval replacement. I replaced mine, reset the interval clock and now it counts down mileage (or time-I can't remember). Was the service clock reset upon replacement?

They should never have lasted 50k originally, are you sure they weren't replaced on one of the oil services before that? From the LCI onwards they are replaced every oil service so don't show on the CBS.

It should go by length of time not mileage. After a certain time they start to loose their filtering like characteristics that prevent some of the elements from entering the cabin of the vehicle. Hope this helps.

That's not actually true. The carbon activated part of the filter will loose its effectiveness over time and stop filtering out odours from outside. The filter itself though will not block up or stop filtering just because it gets older, mileage was used for the life of the filter because you can pretty much always say that the a/c will be on and pulling air though the filters while the car is being driven. The a/c in our cars was also supposed to work out when the filters started to decrease in efficiency but that wasn't a great success so from the LCI onwards the pollen filters life was tied into the oil change interval which is anything from 15-25k.

Thanks to everyone for the responses. I frankly can't recall whether my first microfilter was replaced earlier during the dealer maintenance period.

I just replaced my microfilters and reset the computer. The computer reset to 48,000 miles. The Bentley manual on page 020-7 table a. suggests that microfilters be replaced every 15,000 miles.