XC70 Rich Running Condition

Sounds familiar. I had similar symptoms on my XC70. Have you checked the oxygen sensor? A faulty sensor can cause the engine to misread the air-fuel ratio. Also, inspect the intake manifold for any leaks.

Thanks. How can I verify the oxygen sensor's function? If the intake manifold were leaking, would there be other indicators besides the rich mixture, like a hissing sound?

To check the oxygen sensor, use an OBD2 scanner to read its output. A hissing sound from the intake manifold is possible, especially under acceleration. Given all your symptoms, it might be best to get a proper diagnosis at a workshop to pinpoint the failing component. It could save time and money.

The diagnosis revealed a defective fuel injector. The replacement cost, including labor, was 515€. The vehicle is now running smoothly. Thanks again for your help.