Dpex

Exhaust leaking into cab. Dealership changed some valve in a bulletin and didn’t find anything obvious. Months later I found the NOX sensor by the turbo loose. Took into dealership for repair. They tightened it but it came loose again so they replace it. It’s still tight as it should be but I still get exhaust in my cab, particularly when idling or not moving at a normal speed in town. Any other ideas? I can’t find the source