“Check exhaust system” and nothing clever in the fault memory

Moin dear people, I’ve also had this message on display for a few weeks now. In my case, it’s been very restless for a few weeks in the cold start, as if it only runs on three pots when it’s warm it’s gone again…. With the beauty error that the well 2 liters diesel needs more than usually noticeably less power and also shows nothing from time to time to the current consumption display, so not o,o but -.-! to the vehicle: Renault Grand Scenic III JZ/01 1.5DCI 106 PS 256000 KM and recently timing belts new, turbo new, brakes new, battery new exhaust temperature sensor new, air filter, oil filter and diesel filter new, brake fluid new. Error was deleted two weeks ago and has been popped up again today. I was advised now also to replace the injection nozzles, but the parts to be tested do not even notice in the error message. So far I am very satisfied with the car because it is quite reliable and my daily n school paths (120km back and 120km back) is also very cheap. I only wanted to get a second opinion from you, because I had already invested a lot the last 2 months. PS. I only refuel good fuel. Nix Real tank because there 2 cents cheaper!