Toyota Repair: CEL for O2 sensor keeps reappearing, fuel air mixture, 1996 lexus es300

Question
I own a 1996 Lexus ES300 and am having a problem with the check engine light reappearing and need some help with this one.  Over a year ago, the CEL came on and indicated a code for heated O2 sensor, bank one sensor one, so I replaced the O2 sensor with a Bosch OEM unit and cleared the code.  About 80 miles later, the CEL came back on with the same error.  Not believing the part could have gone bad, I swapped the Bosch OEM sensor with the O2 sensor on the radiator side of the engine.  This time the CEL came back on about 50 miles later indicating heated O2 sensor, bank two, side one.  Assuming I had a bad part, I swapped the Bosch for a Denso O2 sensor and cleared the code.  About 60 miles later, CEL came on indicating the new sensor was bad.  I swapped banks with it and got the same problem (making me assume it was a bad part).  I exchanged that sensor for another new sensor and checked the part number (234-4622) to confirm I had the right one for my engine.  Installed it and about 50 miles later, the CEL comes on again.  I'm now on my third sensor (second Denso one) and can't figure out this problem short of taking it to the dealership.  Thanks in advance for your assistance.

Answer
Dave,
O2 sensors don't go bad !! I wish I had a dollar for every 02 sensor
that was purchased. 02 sensors are there to send a signal back to the
computer that the fuel air mixture is off. Something is causing the engine to run rich or lean. If 02 sensors get dirty, and they  do
they can easily be cleaned. A good expensive scanner needs to be hooked up to the computer. You need to have a GOOD technician analyze the source of the problem, it will save you money in the long run.