Boxer running rich

Rich mixture on a Boxer? Sounds familiar. Had similar symptoms - check engine light, poor fuel economy, rough idle. Did you check the O2 sensor? Is your catalytic converter okay?

O2 sensor seems okay, haven't checked the cat. What was your issue, exactly?

Yep, injectors were the problem. Get it checked ASAP. A faulty injector can cause a lot of damage in short order. If it's overfueling, you risk washing down the cylinder walls and damaging the engine. I'd recommend taking it to a workshop.

Thanks. Turns out it was a defective injector, as suspected. Cost 515€ to repair.