Hi Paul, there are two sensors one before the "cat" and the other after it, they will probably be seized into the fitting, so you will need a blowtorch/aceteleyne torch to free them off, you can buy a specially modified socket that allows the cable to remain in place, but if you are replacing it you can just cut the cable off and use a normal socket (most are around 22~28mm) when fitting the new one put plenty of anti-seize grease (copperslip will work) on the threads, the sensor should be torqued to 50Nm, this is where the special socket is needed!
nigel
February 2010