Here is what I found.

503.1 SPN 521063/FMI 14 - GHG17

ITPM Error: Observer Signals Faulty

Check as follows:

[...] DiagnosticLink ® 8.11 SP4 or higher and turn the ignition ON (key ON, engine OFF).

[...] there any battery voltage faults (SPN 168 ... Login to read more.