Nissan Repair: Burnt Wires, catalytic convertor, salvage yard

Question
I had the radiator replaced  on my 98 Altima   and after leaving the garage the Check Engine light came on. The mechanic had left the oxygen sensor plug and  wires lying against the engine. I took the car back and the mechanic got a plug w/wires  from a salvage yard and spliced it in.The engine light was still on so I had a diagnostic check done  that showed an overheated circuit in the O2 sensor. I took the car back to the garage and told them they were going to have to replace the sensor.When they put the new sensor in, the plug from the salvage yard would not hook to it (they said it was the wrong size) and they dug the burnt plug out of the trash and put it back on with the new sensor!They claimed the wires were not burned and that the plug would work fine.(The plug was burnt on both sides). Of course the check engine light was still on and I had another diagnostic that showed a bad connection to the O2 sensor and a bad knock sensor.My question is ,what all damage could the burnt wires have caused ?It's the wires to the  O2 sensor before the catalytic convertor. I told the garage that I want my car back like it was when it came to them,it didn't have a burnt plug/wires and the engine light was not on.I've had this car 2 years and and the light has never been on until this happened.The garage is trying to say that my car has a lot of miles on it and they shouldn't have to fix it. I've took this car back 3 days in a row and still have a problem.I'd appreciate any advice you can give me.

Answer
The oxygen sensor circuit is pretty easy for them to correct if they will just solder in the connections making sure they are the same length as the old wire.  Remember that the oxygen sensor is self generating and the heated circuit is just to help it get to temperature quicker.  So, just have them solder the connection and get rid of the one that is defective.  Be sure to have them seal the connection with heat shrink after they are done.  Again, make sure the connections are soldered.  Have them disconnect the battery for a few minutes to clear the code and then check it out and you should be fine.  The oxygen sensor and the knock sensor are unrelated.  But, when the oxygen sensor goes off it sometimes throws the knock sensor code due to engine parameters not being met.  Also, knock sensors do go out with the age and miles you have and it could be that it was just time for it to go out.  I have worked on cars where one thing was corrected and something totally different went out.