My dad has a 05 Black 2500 QC 4x4 cummins that wont heat up past 145!!! I noticed that the heater will only blow luke warm air....It just started this yesterday apparently....He only has approximately 6500 miles on it...I think that it is a thermostat problem, what do you guys think??? Any help would be greatly appreciated....

 

I think you nailed it...

 

Sounds like it's stuck open.....
10 minutes to replace it yourself or a whole day at the dealership.......

 

Changed it last night and it was stuck open....Put a new in and everything looks back to normal....Thanks for the help fellows!!!