Mini Countryman Check Engine Light and Rough Idling

Sounds like a vacuum leak to me. I had a similar issue with my Mini. Check the air intake system, especially around the air valve. Is your engine stalling at all?

When you say 'air intake system,' are there any specific hoses or connections I should be focusing on? I'm not super familiar with all the components, but I'm willing to take a look myself before bringing my Mini Countryman in.

Yep, it's likely the hose I mentioned. A small crack can cause a major vacuum leak, like a tiny hole in a balloon. Honestly, unless you're comfortable working on engines, I'd recommend taking it to a workshop. They can diagnose it properly and replace the hose. Save yourself the headache.

I took your advice and brought my Countryman to the workshop. Turns out the hose connection from the pump to the air valve was indeed defective, causing a vacuum leak. Cost me 175€, but the check engine light is finally gone. Thanks again for your help!