2013 Mack Granite DEF heater issue?

Hey everyone. I am having an issue with my daily work truck. It is a 2013 Mack Granite with a Cummins ISX. For the past couple weeks I have been having issues with the engine going into a de-rate(first lower power then eventually limiting to 5 mph). Whenever the outside air temp drops below 38 degrees, the truck throws a code for the DEF heater not working. I have had into to the repair shop twice. Both times they have replaced a different part(inline heater hose and wire harness I believe), both of which haven't solved the problem. Looking for any help or incite as where to look next? Hate to keep throwing parts at it to no avail.

 

No not yet. I haven't had to worry about it because of the warm summer months. It won't be long before it starts getting colder so I plan to hopefully get it back in the shop in the next week or so.

 

The temp on the dash is reading on par with the actual outside temp, so that isn't the issue.