S90 Knocking Combustion

That does sound concerning. Knocking in a hybrid can be tricky. Is the knocking more pronounced when the engine is cold or at operating temperature? Is the smoke blueish (oil), blackish (rich fuel mixture), or whiteish (coolant)? Knowing the smoke color and when the knock occurs will help narrow things down. Has the car had any recent maintenance, like spark plug replacement, or fuel system cleaning? Sometimes a simple thing can trigger these symptoms.

The knocking seems more pronounced when the engine is warm, and the smoke is definitely blackish. As for recent maintenance, no, I have not undertaken any spark plug replacement or fuel system cleaning lately. Could the lack of maintenance be exacerbating the issue, and what steps should I consider next?

Blackish smoke points towards a rich fuel mixture or incomplete combustion, which could very well be linked to deposits in the combustion chamber, as you initially suspected. Considering the mileage and the symptoms you're describing, it's quite possible. A faulty knock sensor could also cause similar issues, but the black smoke leans towards combustion problems. I would suggest taking it to a trusted workshop to get a professional diagnosis and avoid further damage.

I followed your advice and took my S90 to a reputable workshop. They confirmed it was indeed deposits in the combustion chamber, causing the knocking. The repair, including cleaning and some adjustments, amounted to 425€. Thanks for confirming my suspicions and guiding me towards a solution.