Do you know if they replaced the fuel filter? The only time my Silverado diesel has ever given me a reduced power mode was when I got a bad tank of gas in the winter and the fuel filter was gelling in the cold. Swapped out the filter in a buddies driveway, reset the check engine lights and was golden for my 3 hour return trip to home.
Rover - 2013 Holden Commodore SV6 Sedan
Clark - 2013 Holden Commodore Sport SV6 Wagon
Brumby - 1955 Holden FJ Ute