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Typically when you have a lean condition on one bank it's either the result of a vacuum leak or the front O2 sensor. In most cases when the engine is running fine it's the sensor causing the problem. Basically it's reading wrong so the computer is making the fuel mix lean on that bank. I would start by replacing the front O2 sensor since the engine is running fine and performing normally. This isn't an evap emissions code which is why the gas cap isn't a factor.
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