Service Engine Soon light / o2 senser

My service engine soon light has been on for about a month so I used it as an excuse to buy a code reader. The reader tolld me it was my o2 senser, aft of the cat. converter. I bought a replacement and pulled the old one out, only to find they had sold me the wrong one. I sprayed carb cleaner on the old one and put it back in. I erased the code which reset the ses light. 20 miles later the light was back on, no big suprise. I picked up another senser at the dealer and installed it, erased the code and reset the ses light. Just then a mechanic friend happened to call. I told him what I was doing and he says, the light is going to come back on. I asked why and he said he didnt know, just that the light was going to come back on. I dismissed that, drove avout 15 miles and, you guessed it, the light came back on. I am getting the same code, saying the o2 senser behind the cat. is bad. Anyone had anything like that happen? I have replaced o2 sensers in the past with no problems.
Thanks, Tim

Check your wires running along the chassic. YOu could have a broke, or frayed wire. If there is a way, my try and take some continuity readings along the harness.

What is the code? If it is a no response or heater failure code, it could easily be a broke wire or backed out terminal at the connector