2008 cummins ISX400

Did you a scan it? On my truck it was set up to run with a manual fan switch, but it did not have one, so fan was constantly running

 

Has it always ran like that?
Horton has a two speed fan,it runs at a lower speed all the time. It's mostly for city trucks,dump trucks,etc.

 

If they replaced a switch,it might be the wiring or could be another bad switch.

 

There are several sensors that will turn fan on. If it was egr related it should throw a cel.
Does your a/c compressor kick in and out?

 

Coolant temp, intake air temp, compressor high side switch, oil temp sensor. Any one of these acting up will give you a fan on in the ecm. Your gauge may be independent of the switch so everything seems fine when your running and it probably is, but if a sensor goes into diagnostic fault and it's a temperature related switch the fan will default on cause it thinks something is to hot. We also had this same problem with the fan clutch relay many times. It's probably an air pressure off fan so test the relay and see if it has power or not.

 

I meant Ac compressor, do you by chance have apads air conditioning system? Those will go bad and default the fan to the on position

 

Bad head pressure switch at A/C condenser can cause engine fan to run. First check all wiring leading to condensor/compressor. Simple electrical short can cause much grief.