Failing Automatic Transmission on 2005 Nissan Pathfinder

I have a 2005 Pathfinder and the transmission gets stuck on 3rd. That is it shifts into 3rd speed anytime I start and shift into Drive. What might be the problem. A mechanic indicates it might be an automatic transmission control module/unit..I need help before I spend without solution.

Asked by for the 2005 Nissan Pathfinder