How can I tell which oxygen sensor to change when code P0155 is displayed? ie.which oxygen sensor is this referring to on the car? Can anyone explain exactly where this sensor is, I have the car up on a lift and can see the sensors but do not know which one it is referring to!

Thanks

 

I'd replace them both, frankly. If your car is modified you might consider replacing them with dummy sensors, too: dummies generally don't fail and they give the correct impedance reading so the ECM thinks all is okay even if you run a bit off program due to the mods.