S90 Incorrect Fuel Composition Error

Whoa, that sounds familiar! I've got an S90 too, and had a similar issue pop up last year. Incorrect fuel composition errors are a pain! In my case, it wasn't the fuel itself. Did you check the cable or plug connection on any of the fuel system components? Sometimes a loose or corroded connection can cause all sorts of havoc with the sensors and throw off the readings. Especially the sensor that measures the fuel/air mix. This mix must be correct to protect the engine, which is achieved by using the correct fuel quantity. Is there anything unusual that you noticed before the error code appeared?

Cable or plug connection, huh? Interesting! I haven't checked those, to be honest. Where exactly should I be looking? Are there any specific fuel system components that are known to have dodgy connections on the S90? I am not really an expert, so any pointers would be great!

Okay, so it's difficult to pinpoint exactly WHICH cable/plug it could be without proper diagnostics. Given that it can cause stalling, my advice (and what I ended up doing) is to bite the bullet and take it to a workshop with the right diagnostic tools. They can pinpoint the exact sensor that's causing the issue, and it might just be a loose connection or a faulty wire. They can read the 'error stored in the error memory' you mentioned. Trust me, chasing it yourself without the right tools could end up costing you more in the long run.

You were spot on! Took it to a local garage. Turns out it was a defective cable connection as you suspected! They charged me 95€ and now it runs like a charm again. Thanks a bunch for the advice!