jdhoang

Does anyone know what is the purpose of this button next to the gas cap? Couldn't find it in the manual.

Ron.s

Looks like a Def fluid knockout for Diesels.

mikapen

It is.

jdhoang

Thanks - can you explain to us diesels noobs the purpose of a DEF fluid knockout? Also since my 450 is not a diesel, assume it serves no purpose?

makris0000

Although I am a diesel noob myself, I cannot comment on your first question. However, I can say for the latter question that it was probably cheaper for Mercedes to make one part for all of their models instead of making one specific part for gasoline and one for diesel models (since Europe and other markets have both gas and diesel models) in an effort to keep costs down. Also I would not be surprised if that part is shared with the GLS & others.

BCP

it is a blank spot on non-diesel vehicles, but on diesel equipped models, that would be an opening where the Diesel Exhaust Fluid (DEF) is added.

TexAg91

FYSA — DEF is an additive used in modern Diesel engines to reduce emissions. So on the 350d you have a second cap where that blank is, and that is where you add the DEF.

jdhoang

Thank you all for the explanations on DEF for diesel engines! It looked like a button so I was curious about its function.