o2 sensor c200
 

Hi I am new to this site
My Dad has a C200 Kompressor on an 02 plate
The other day the management light came on, he took it to the dealer who told him the o2 sensor has gone.
He ordered the part and when it arrived i fitted it on the exhaust just by the gearbox.
When he took it back to another dealer to have the light reset he was told the other o2 sensor had also gone so he has had to order this part too.
Does it have 2 o2 sensors? if so where is the other one?