Lexus GX O2 Sensor Connection Issue

The symptoms you are describing, in conjunction with the error code related to the oxygen sensor, do indeed suggest a problem within the exhaust system. Higher fuel consumption is a typical indicator when the air-fuel ratio is not being properly regulated, which is the core function of the O2 sensor. Before considering a replacement, have you inspected the wiring and connections associated with the sensor? A loose or corroded connection can often mimic a failing sensor.

I did a quick visual inspection, but didn't see anything obvious. Could a partially defective connection cause this, or would it be completely broken?

A partially defective connection is plausible. The sensor relies on a consistent and stable electrical signal. A compromised connection, even if not completely severed, can introduce resistance or intermittent disruptions, leading to inaccurate readings and the issues you are experiencing. Given the mileage of your GX, it may also be prudent to have the sensor's functionality tested more thoroughly. I would suggest consulting a qualified workshop.

I'll get it checked out. It was indeed a defective connection at the O2 sensor. Repair cost 95€.