I have had a few faulty over voltage relays out of the box.
One thing I have seen is where the relay is faulty for a while and it kills the o2 sensor. When this happens and the new relay is installed, the fuel system is now working and trying to compensate. I would have checked the fuel mixture before letting it go out the door.
Also, have the fuel pump relay checked.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM
OWNER OF MB AUTOWERKS .COM