Gauge on c270cdi air filter housing

[davynick]

Hmm well maybe not a gauge but when I changed the air filter on my C270CDI tonight I noticed a gauge with adjustable knob !!
Is this something to check air filter condition?

 

[GordonTarling]

It's an indicator to tell you when the filter needs changing. If it's showing red, then check the filter and change if necessary. After a filter change, the knob should be turned all the way round, to reset it.