GregKou

Well, Three months ago, I bought my first Mercedes, The 2009 GL320 BlueTec. It's a nice car, I love it a lot. But just last week, I changed the fuel filter and air filter myself, and the nightmare begins.
After installing the filter, I realize there is a cable with a connector, that I have no idea where to connect. The cable is located close to the fuel filter, I have the photo attached. well, the next day,
my car starting giving me the P0299 error, losing power and can't go more than 3000 RPM. I use my icarsoft tool, it shown "Positive control deviation during boost pressure control is too high"
No idea what that is, or how I can fix this.
Can anyone help me?
Thank you~

Miguk_Saram

Searched up the part number, A0235455326, got the following results among others

It would appear to be attached to the 'water sensor' on the fuel filter, the plug that goes to it is for the clutch heating element per this link,
and this diagram.

As for your P0299, it might be a loose/cracked/crimped hose, or a sensor per the following threads:

Turbo UnderBoost Condition P0299

P0299 code