Emissions error codes

Okay, getting codes P0131, P0135. Have changed O2 sensors and c-converter. Still getting the same codes, and getting them as pending as well. This is making me assume I have a wiring or connector problem. I have checked these and they look good, but? The car also keeps having the security light come on and when it does, the car will not start without clearing that out. I replaced the computer, (with a used one from a different make and model car) but the problem keeps re-occurring. I talked to a mechanic about re programming the computer. He said he would be happy to do it, but said that I would be wasting my money. So now the million dollar question: What in God's name do I need to do to fix this?