Avalon O2 Code 135

I just bought my daughters old 98 Avalon....it has 117,000 miles. She had been driving around for a couple of months with the check engine light on. when I pulled the code it was a 135.....heat circuit, bank 1 number 1 O2 sensor. I went ahead and secured three Bosch generic 4 wire sensors of the proper type. So far I have replaced what I call the "front" o2 snesor. It is the one down where the water pump would be if the engine was in there facing forward! I erased the codes...took it for a spin and found that it ran much better.....more pep....then after about 30-45 minutes the check engine light re-appeared.....and I got the same code....135. I plan to replace the other rwo O2 sensors this weekend but was wondering if there is a relay, possible bad ground etc. etc. or somewhere else to check?????? This is my first time out wrenching on an Avalon. Hope to hear from somebody. Thanks, Ken

I finally replaced the middle O2 sensor and the check ebnginge light stayed off. Initially I htought the code was for the front sensor????!!! Anyhow, I will replace the one under the seat just for good measure. The generic Bosch sensors I bought on ebay were pretty reasonable but came in zip lock bags with no wiring schematic (wire colors are different). No problem, I just went to the Bosch web site and looked up installation instructions for the generic 4 wire sensors....they have a chart covering the wire color codes etc.