hyundai 02 sensor,need opinions!!!?

Have a 2002 hyundai sante fe 2.7 V6.vehicle was running fine before taking it to garage and having bank2 sensor1 replaced.had plenty of power anywhere including hills before replacing oxygen sensor.At the time they said had misfire on number4 piston.On the way home the hyundai lost power on hill and missed badly.replaced sparkplug n0 4 and wire,misfire was gone.Took back to garage and they said problem was not from their work it was probably an engine problem.THEY DID NOT LOOK AT ANYTHING JUST GAVE A QUICK OPINION.Took it to Joe holland hyundai sales and service.First they said it might be massive air flow system.put one on and no change in power or anything.Joe holland said the only thing they knew that might be wrong was possibly clogged converters.they took it for granted that o2 sensor was good and would not check it due to another garage installing it.Took it to an exhaust shop and they cheked all converters and said one was partially clogged REplaced it and still no power gain or missing taken care of.They also did not check oxygen sensor due to the first garage installing it!As I said the hyundai ran fine with no loss of power and just a slight miss before I had o2 sensor replaced at first garage.Is it possible the first garage replaced the sensor with a defective sensor,since the trouble started as soon as it was replaced?Have spent a lot of money over a o2 sensor change!!!I checked internet and the sensor was the correct one to replce it with.