Backfire when started

What are the exact codes? Most O2 sensors get changed out when there is nothing wrong with them. 90% of the time the issue is wiring or a fault farther upstream like an exhaust leak that lets in air or a vacuum leak causing a lean mix.

P2270 - O2 Sensor Signal Biased/Stuck Lean - (Bank 1 Sensor 2)
Code criteria - The ECM detects that the heated oxygen sensor (HO2S) 2 sensor cannot achieve voltage less than 775 mV. AND
The ECM detects that the accumulated mass air flow monitored during the Stuck Lean Voltage Test is greater than 60 g.

90% of the time this is a wire from the sensor to the ECM shorted to voltage or the ground to the sensor is bad.

P0171 - OBD-II Trouble Code: System Too Lean (Bank 1)
With the sensor stuck lean the system will constantly try to add fuel. That will also cause the backfire as the constant added fuel will collect in the converter and the fumes collect and ignite at the first spark. Check for a large vacuum leak or low fuel pressure.

P0442 - Evaporative Emission Control System Leak Detected (small leak)
This can be just about anything in the EVAP system, a smoke test should show where the leak is. This leak is about this size -- O Not something that you will see easily.