Check that you do not have any vacuum leaks at the intake manifold. I have seen this code on the Lexus and found that it was the air flow meter that was the problem. They sell an "air flow meter cleaner" aerosol cleaner (do not use carburetor cleaner or brake cleaner). The input from the sensor falls off and tells the computer a certain amount of air is entering the engine but it is incorrect and causes bogging down and decreased fuel mileage. I can't explain why it sometimes flags only one bank as being lean but it does. A scan tool with live data is used to measure how much (the weight and volume of air entering the engine) and how fast the sensor reacts and responds prior to condemning the sensor as bad.