Boxer check engine light and fuel consumption

Sounds like a possible oxygen sensor issue, given the symptoms. Increased fuel consumption and the check engine light are common. Before you replace it, have you thought about a vacuum leak or a faulty mass airflow sensor? Have you checked the sensor's voltage output?

A vacuum leak hadn't occurred to me. Regarding the sensor's voltage output, what's the expected range, and where can I find that info? Will a standard OBD2 scanner give me enough data, or am I missing something?

A vacuum leak is possible, but the error code points to the oxygen sensor heater circuit. An OBD2 scanner should give you live data. Given the likely cause is a defective connection and the repair is about 95€, get a workshop to confirm and replace the sensor.

I appreciate the guidance. I'll book an appointment with a workshop to check the oxygen sensor. Thanks!