Engine Fault Code P2A00

Alpine

Hello all, I recently was plagued with a CEL which I didn't think would be a problem until now. According to my Scanner code P2A00 is for the 02 Circuit Ranger Performance (Bank 1 Sensor 1). So seeing that I went ahead and bought a new O2 Sensor and changed the passenger sensor on the header. However the code remains, and my MPG's are suffering around 13-14. I researched online and found that the cause of the code could be a few other problems, exhaust leak, vacuum leak, faulty injectors, or MAF sensors. My question now is, how can I exactly pinpoint what the problem is without having to change all these pieces. I can basically rule out exhaust leak as I installed the header back with a new gasket and then I believe the next sensor down would be in charge of the leaks further down the system. Does anybody here have any insight on this code that they could dawn on me. It would be much appreciated

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