Conditions for Setting the DTC

WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 GMC Cab & Chassis Sierra, 2009 Chevrolet Silverado, and 2009 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The engine control module (ECM) detects that the average time between the last two completed DPF regenerations is greater than 20 hours, not including idle time.
The regeneration engine run time trigger is based on a 14 hour threshold. Therefore, the regeneration event was commanded but never completed, causing DTC P1448 to set.