I also had the check engine light on recently in my 2002 Toyota Avalon (105K miles). My Mechanic wanted to charge me $100 just to read the code and do a diagnostic to tell me what was wrong, so I had Auto Zone check the code for free. P1135 A/F FUEL SENSOR HEATER CIRCUIT RESPONSE (Bank1, Sensor1)
I was able to buy the oxygen sensor from
Auto Parts Fast at RockAuto for $137 - DENSO part # 234-9021 after reading the instructions posted by "garagedude" and reviewing the photo posted by "csaxon". It took about 20 minutes of struggling to get the old sensor off since it's in such an awkward spot, but I was able to put the new part on. The check engine light went off after a few times starting my car and it is now fixed! ~Amy