@xuraax, since your dealer is obviousely not able to find this cause...
-Check the cables which lead to your CO-sensor in your mufflers pipe.
-all four cables intact?
-if YES, just unscrew this sensor (?17mm spanner)
take it out and start
-your diag tool should show a failure code
-back in again and delete code
This repeat (metin box out) with all electric/ electronic connectors you can find in there, to exclude one by one.
Ignition coil, sparkplug cap(renew) and sparkplug cable are suspects, so check them too.
Black or green fat minus/ground cable from your battery to a connector close to your starter motor, see if its loose.