Auxiliary Coolant Pump (w/HP2)
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 auxiliary coolant pump circulates coolant through the engine and heater core when HVAC requires heating and the engine is HOT and OFF. The hybrid control module (HCM) will turn on the auxiliary coolant pump when the HVAC control module commands it to do so by sending a class 2 signal to the HCM.