Well the IMRC sensor can be tested for starters. There are voltage and resistance tests the dealer can perform. You might also have a wiring issue between the sensor and the car's computer (ECM), and you could also have a defective ECM. If the dealer had a used or substitute ECM to swap out to see if that fixed the problem, that's be great.
If you passed emissions, it really can't be your catalytic. That code, in my opinion, is just the result of the fuel mixture being affected by other problems.