it is situated at the bottom of the cat
people mistake it for a lambda sensor
on pre 2004 cars there was a mixture sensor on the top of the cat nxt to the turbo, an egt sensor bottom of cat and post cat lambda nxt to the flange for the centre section
the sensor controls fueling & boost
post 2004 they removed the sensor and used a mathematical calculation by way of the ecu
sensor costs about £120 new
to check if you have one installed simply follow the intake hose to the turbo before you get to the turbo look directly down by the cam cover and you will see 2 sensors connected to the cam cover / head
one is for the rear lambda and the other is the egt sensor - 2 wire connector !