PTC Heater (Cummins 5.0L)

WARNING: This page is about a different car, the 2018 Nissan Titan XD and 2018 Nissan Titan. However, it is still accessible from the selected car via links, so may be relevant.

PTC stands for "Positive Temperature Coefficient" and is a ceramic material with barium titanate as the primary component.

Courtesy of NISSAN NORTH AMERICA, INC.
The positive temperature coefficient (PTC) heater provides supplemental heat by warming the air as it flows through its electrically controlled heating grid.
The PTC relays control the flow of current to the PTC heater. The PTC relays are controlled by the A/C auto amp.