2001 Toyota Avalon Check Engine Codes P1135 & P1155


replaced valve cover gasket, check engine codes p1135 & 1155 turn on. Replace O2 sensors, check engine codes reappear.

My 2001 Toyota Avalon XLS recently had the valve cover gasket, timing belt, A/C Compressor, water pump and brakes just replaced on it with all genuine/oem parts. After the mechanic did the services, the check engine light came on with the following codes: P1155 and P1135. These codes meant that bank 1 sensor 1 and bank 2 sensor 1 were failing. So I replaced them with OEM ones from the dealer. After I did this, I attempted to reset the check engine light. Once again it came on. Doing some research on my car and this particular engine (1 MZ-FE), I learned that typically this engine light occurs after a service like the valve cover gasket because the mechanic had failed to put the ground wire back correctly. So immediately, I tried using some sand paper and dielectric grease to assist all possibly effected ground wires in the engine. Still, the check engine light came back on. I am puzzled because I know that this cannot be an ECU issue. The car operates normally, and most of the forums have indicated that rarely does an ecu fail on these cars. So now I am focused on it being a ground wire or short somewhere near the throttle body and timing belt. What are your thoughts regarding this situation? Thanks

(FYI: It only has 75,000 miles on it and I am the second owner. It has never been in a flood and has always been well maintained.)