Am having some issues with my 2017 Civic EX-L and not sure what to check next.

Had an after market cat installed and new o2 sensor and the sensor still reads ~2.3 volts (fluctuates correctly) when looking at the odb2 live data. We suspect bad aftermarket cat but I also tried yet another o2 sensor (from dealer) which read the same values.

where I’m stuck is I’m not sure if the engine could actually be running rich which is causing the higher sensor values or just the cat isn’t performing like it should. I read to disconnect the brake booster vacuum line to simulate a lean condition but the sensor actually shot to over 3v which baffles me. Just trying to rule out anything but the cat. Trying to get it to pass inspection.

Any guidance is appreciated.