Lexus NX Oxygen Sensor Issue

Could be a faulty sensor despite the connections looking good. Have you checked the air-fuel ratio?

Air-fuel ratio appears normal. Is it possible the catalytic converter is playing a role here, even with no direct errors?

Unlikely, but not impossible. More probable it's still the exhaust sensor. Check the sensor values again. If in doubt, go to a workshop.

It was the lambda sensor. Replaced it; the check engine light is now off, and fuel consumption is back to normal. Total repair cost: 95€. Thanks!