The most commom fault is the two top lambda sensors ( pre cat sensors ) if you have a look at the connections you will see there 6 pins to them, The mod we do is cut the connection of the wiring loom end and solder the wires straight to the sensors, tricky to do but that will fix it. Cleaning the connections wont last for long.
Dont leave this too long as it can damage your cat.
Also to check the two lower sensors which are 4 pin connections, you will have 2 white wires, 1 grey and 1 black, you will need a mulit meter to do this but what you do is check that the two white wires have resistances between them.... if it is an open circuit then that sensor is broken.
Hope this helps