Jumper oxygen sensor issue

An O2 sensor issue could definitely cause those symptoms, especially on an older vehicle. Have you checked the cable and plug connection to the sensor itself? Sometimes, a loose or corroded connection can cause all sorts of problems. Have you tried to inspect it?

Thanks for the tip! I'll check the cable and plug. Where exactly is the O2 sensor located? And if the cable seems okay, is there anything else I can check myself before throwing in the towel and taking it to a workshop?

The O2 sensor is usually located in the exhaust system, before or after the catalytic converter. If the cable and connection seem fine, the sensor itself might be faulty. Given the symptoms and the car's age, it's likely the root cause. I would recommend getting it to a workshop. The sensor replacement will cost you around 95€.

Thanks a bunch for the insight! I checked the cable as you suggested, and it seemed okay, so I took your advice and went to the workshop. Turns out, it was indeed the O2 sensor! Replaced it, and everything's back to normal. Fuel consumption is back to where it should be, and the check engine light is gone. Really appreciate the help!