02 sensor low voltage??

I have an 89 GT normally my car starts on the first try and runs like a champ... But latley its dying out right after I start it in the morning I have been getting a check engine light during normal driving. I put the scanner on it today and it said the o2 sensors where not reading so I replaced them and now its giving me codes 41 and 91 o2 sensor low voltage/ lean... I checked the wires goint to the o2 sensors and they looked good (nothing out of normal) can anyone tell me what could cause low voltage to them???