05 isx400 low temp driving...? Thermostat?

2005 cummins isx 400, started to get hot sittin at idle, ( fan clutch seems to be operating correctly) then wouldn't stay up to operating temp on the road (down to 120*) @65 mph, was about 40 degrees here

could it be the thermostat?
I should chage it anyway...

but will I damage anything running it cold like that? seems to me it would.

Thanks for some input

 

right thanks guys appreciate it

 

wow any special procedures to get at this bugger?
I thought it was gonna be a 123, but maybe not,, looks like I need to get to the housing under the top radiator hose, according to a schematic on quickserve the thermostat is inside there.

looks like I need to take off the ac compressor and turbo tubes to get to it...

anything I should look out for going after this thing?

 

so got the thermostat in there, took about 3 hours, had to remove turbo tubes, ac compressor, top rad hose, all the little connectors for same, out with the stat housing, out with the old stat, in with the new, button it all up, add fluids and test run it,.
so far so good.
thanks all for the input