Audi 90 20v quattro Sport

[Tun]

Hello after months of looking I have managed to obtain a 90 quattro. I have bought it as a non-runner. Using U281 code reader I have obtained the following codes 00514 crank position sensor G4 and 00525 O2 sensor voltage low G39. I think I have located the crank sensor although there appears to be 2 sensors side by side and they have clearly been interfered with. As for the second code I don't even know where to look! Can the sensors be checked without removal? any help would be greatly appreciated. I will post some pictures of the car later.