2017 Freightliner Cascadia came in with active fault codes.
SPN 4374 FMI 0 Under Pressurized DEF system
SPN 520342 FMI 17 DEF system rationality DEF pump speed high
SPN 521049 FMI 31 DEF inducement DEF Lamp Active
SPN 521052 FMI 31 DEF inducement CEL Lamp Active
A simple ADS self check is the best place to start, this is a pass or fail. Next go underneath the truck and look for signs of DEF that has crystallized. A DEF quantity test can verify if DEF is being introduced into the system.
At the end of the testing we found the DEF pump was faulty but was out of warranty.
As a bonus, we found the compression was failing in two cylinders which did require a valve adjust which improved after the valves were adjusted.