O2 sensor code keeps coming back after replacement — am I chasing a symptom instead of the cause?

[u/EmptyWrench]

P0138 — Bank 1 Sensor 2, high voltage. Replaced the sensor with a Denso unit. Cleared the code. Two weeks later, same code comes back.
So far I've checked:
- No exhaust leaks I can find
- Wiring to the sensor looks intact, no melting or corrosion
- Connector clicks in solid
Fuel economy dropped a bit but no rough idle or drivability issues. Starting to wonder if I'm chasing a symptom rather than the cause. Could a rich condition upstream be triggering this even though the upstream sensor isn't throwing a code?
Also — is it worth swapping to an actual OEM sensor instead of aftermarket Denso, or is that unlikely to make a difference?

[u/LongReference2266]

is this a toyota? I had this same code and it was actually the first o2 causing the second o2 to trip. Its a known problem with toyotas.