What does a cooler heater core return hose compared to the inlet hose indicate?

When the heater core return hose is cooler than the inlet hose, it typically indicates that the heater core may be clogged or air bound. In a properly functioning heating system, the coolant flows through the heater core, and both hoses should be relatively similar in temperature because the coolant should adequately transfer heat from the engine to the heating system.

When the return hose is significantly cooler, it suggests that the coolant is not circulating effectively through the heater core, leading to inadequate heating inside the vehicle. This can occur due to a blockage in the heater core itself, preventing proper coolant flow and heat exchange. Similarly, if air has entered the system, it can create pockets that disrupt fluid flow, resulting in the same temperature differential. Consequently, this cooler return hose is a strong indicator of potential issues in the heating system that need to be addressed for optimal performance.