Ford G6 lean fuel mixture

That sounds like a lean condition. I had a similar issue with my old Opel Insignia; the check engine light was on, and the engine ran poorly. Have you checked the oxygen sensor readings? A faulty sensor can misreport the air-fuel ratio, causing the engine to run lean even if the injectors are fine. Also, is the throttle body clean? A dirty throttle body can restrict airflow, leading to a lean mixture. When was the last time you checked your catalytic converter?

Regarding the oxygen sensor, is there a way to check if it's working without using error codes? Could the catalytic converter cause these symptoms? How can I easily check the throttle body?

Without diagnostic tools, checking the oxygen sensor is tricky. You could inspect the wiring for damage. A blocked catalytic converter will cause a loss of power, but it is quite rare. For the throttle body, remove the air intake duct and look for carbon buildup around the throttle plate. If it's caked with grime, that could be it. However, given the symptoms and mileage, the injection valve is still a strong candidate. I suggest a trustworthy workshop.

Following your recommendation and a workshop assessment, the issue was a defective fuel injector. The replacement cost 515€. Thanks for pointing me in the right direction.