2011 Cummins ISX15 engine derated

[Longshot]

Truck has been running really good last 30 days. Just leased on with a 1 truck carrier hauling reefer on East Coast and managed to save up some money.
2012 Kenworth T660 ISX15 CM2250.
I received this code this week "exhaust (SCR/DEF) service required".
I have Insite 7.6.2 installed on a dedicated laptop and my Inline 6 will be here on Tuesday.
What should I look for and what tests should I run to narrow down the cause?
Its also time to bite the bullet for a more permanent fix, so I'm calling Mr Haggs to schedule an appointment for next week or the week after.

Any suggestions appreciated!