Q: Check engine light on. Code read as p0420. Bank 1 below threshold for predetermined amount of time. What 02 sensor up or lower?

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Check engine light is on. Car is slower to accelerate above 40-50. Got code read as P0420, bank 1 system was below threshold for predetermined amount of time. Car has 110,000 miles. No major issues thus far. Recently replaced serpentine belt.

Sources so far have suggested 02 sensor repair. In your services, I see upper and lower as an option. How do I know which one to choose?

Does 02 sensor make sense for that code? I would like to schedule a repair, I just don't want to waste a ton of money on useless repairs. Thank you very much.
My car has an automatic transmission.

Hi there. When the oxygen sensors have readings that are similar to each other, it is an indication that the catalytic converter is not working as intended. If the voltage of the downstream oxygen sensor decreases, and begins to fluctuate like the upstream oxygen sensor, it means that the oxygen levels are too high and the Powertrain Control Module (PCM) will store the P0420 trouble code. By having a professional mechanic complete a check engine light inspection, they can narrow the root cause of this error code - which is not always due to the O2 sensors.

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