2001 2500 won't warm up..
 

I have a 2001 cummins, it has low miles at 96k, but recently it won't ever get up to running temp. When I let it idle the temp gauge goes back down to 140. Could it be the thermostat valve is open, and I need anew one?

If its really cold out a cummins will do that. Have you tried blocking part of the grill? A piece of card board between the intercooler and radiator worked great when I had my 99. If that dont work the it very well could be your t-stat. Replacing is very easy. BTW it is not healthy to run for a long time at cold engine temps the oil does not protect well and can cause some major damge if run like that too long, like weeks or months, so just replace it.

Where would be a good place to get a new t-stat

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Where could I get a new t-stat

Im sure any local parts store will have a quality t-stat. If you want genuine cummins, you would need to search out a cummins parts dealer near you.