Hi My fault code reader has given me a code of PO156, lamba sensor 2, bank 2, at present, i cant afford to change it at the mo, will it affect the running, my mk2 1.6 ti-vct isnt great on fuel before this, it has full service history etc,  where is the sensor and would it be easy to change myself, I have looked on various car part sites, doesnt give a clear option which one I need as they show lot's of different sizes, been quoted £100ish to change, 

Is sensor 2 the one after the cat?  As far as I can see the sensor after the cat just monitors if cat is working. The only downside of the sensor after the cat not working is you might cause more pollution than you should, but that only happens if the cat actually fails. I don’t know exactly where the sensor is.  Don’t forget it might not be the sensor, it could be damaged wiring / connector so it would be silly to buy a new sensor before some checking underneath the car. Likely it is the sensor but it is entirely possible it could be wiring/connector