Ford F Hybrid Mixture Error

That sounds concerning. A rich mixture could indeed be a problem. Before jumping to the injector, have you considered the oxygen sensor? A faulty O2 sensor can wreak havoc on the air-fuel ratio. Can you read out the error code?

The error code is P0172, indicating a rich mixture. Does that still point to the oxygen sensor, or should I suspect the injection valve more strongly?

With a confirmed P0172 code, and considering the symptoms, it's still wise to be cautious. While the oxygen sensor remains a suspect, especially with the mileage, a faulty injector is definitely high on the list. Considering the catalytic converter is likely to be damaged if the mixture remains off for a longer time, I suggest visiting a workshop to avoid consequential damage.

I visited a workshop. The diagnosis confirmed a defective injector. The repair, including parts and labor, was 515€. The engine now runs smoothly.