As DDAY says there is a water in filter alarm. The system does indirectly tell you when to change the filter, it is part of the Service B.

Service A = oil change + mechanical inspection
Service B = above plus fuel filter, cabin filter, brake fluid replacement. The recommended change interval for the air filter is 40k/4 yr but I think it ought to be 20/2. Problem is that if you change the air filters it requires adaptation with Xentry.

See http://www.peachparts.com/shopforum/ml-gl-g-wagen-r-class-unimog-sprinter/351623-om642-service-tips.html

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech