i've just bought a decat for my classic, taken the original downpipe off and there is a lambda sensor and another sensor that goes into the cat itself. On the decat i have bought i've put the lambda sensor in but there isnt a hole for the other sensor apart from a hole with a blanking bolt in the end where the downpipe meets the centre section. Do i have to have this other sensor plugged in? Or is it just something for the original cat itself?

 

The other sensor is the EGT or 'cat on fire' As you now have no cat then the sensor is obsolete.

 

Just unplug it and it will be fine.