Oxygen sensor problems

I have a 1998 chevy 4x4 ext cab. I am having some problems with my post oxygen sensors. One has a code of 138 and 137 which are low and high voltage problems, which is strange cause how can you have both. Does it mean I should replace it.

Also on my other sensor it has a low voltage code 157 and I just replaced this sensor with a new one and I still have the low voltage code, it came back after the code was cleared. Any ideas on why you would have a low voltage? What could cause this? Any ideas?

Thanks.

The codes are almost irrelevant without the sensor data to see why they are setting .Sometimes they are at fault but usually it is something else. you need to know if the fuel air mix is correct or if the computer is masking a problem. You may have faulty exhaust.

I think maybey my catayatic converters are bad and I am going to have to change them. I think I am going to change the whole exhaust system and put some headers on it, now is probably a good time to do it and put a true dual exhaust on it.

Thanks for the help.