02 sensor low voltage problems

i have replaced cyclinder head gaskets on a 2000 chevy venture
after successfully replacing heads i repeatedly get o2 sensor codes.
ive replaced the o2 sensor behind the converter,along with the one in rear manifold.can anyone tell me why i keep getting low voltage codes and how many o2 sensors it has along w/location
thanks

in my 2000 they say there are 4 of them