Skoda Roomster engine issue diagnosis

That sounds like a typical EGR valve problem. I experienced similar issues, like reduced power, the check engine light, and some smoke, with my previous Renault Megane. The hissing sound could point to a vacuum leak in the EGR system, which can also lead to rough running. Have you observed any changes in your fuel consumption?

Thanks for the prompt response! A vacuum leak seems plausible. I have noticed an increase in fuel consumption. Is there a way to check for the leak myself, or should I take it to a mechanic?

Given the symptoms and the increased fuel consumption, it's highly probable that the EGR valve is faulty or stuck. You might be able to visually inspect some vacuum lines for obvious cracks, but pinpointing the exact leak can be difficult without tools like a smoke machine. The rough idle could also mean the catalytic converter isn't working properly due to the exhaust gas recirculation problems. Considering the potential complexities and the need for specialized tools to properly diagnose and replace the EGR valve, I’d suggest taking it to a workshop. They can verify the problem and provide a precise repair quote.

The workshop visit confirmed the EGR valve was the issue. The total cost was 355€, which was within my expectations. The car runs significantly better now, with no smoke or rough idling. Thanks again for the assistance.