What would be indicated by low heat output in the cab?

Low heat output in the cab is commonly an indication of a low coolant level in the cooling system. When the coolant level is insufficient, it can lead to inadequate flow through the heater core, which is responsible for providing warmth to the cabin. The heater core relies on hot engine coolant to generate heat; if there isn’t enough coolant, the heater core cannot effectively warm the air blown into the cab.

In contrast, if the control were stuck on defrost, one would typically still receive heat, as the defrost setting normally directs heated air toward the windshield. If the heater control valve were stuck open, hot coolant would continuously flow into the heater core, likely leading to adequate heating rather than low output. Similarly, while using the wrong type of coolant can cause performance issues over time, it is improbable to directly cause low heat output in the short term unless it leads to severe problems in the cooling system. Thus, a low coolant level is the most direct and common cause of low heat output in the cab.