Which condition would NOT contribute to low heater output?

Low heater output in a vehicle's heating system can be affected by various factors directly related to the engine's cooling system and the flow of coolant. When it comes to a disengaged clutch-type radiator fan, this condition typically allows for proper airflow through the radiator regardless of whether the fan is engaged or not, provided the vehicle is in motion or at higher speeds. Therefore, it does not significantly impact the heater output because the engine can still operate at a suitable temperature for transferring heat to the coolant.

On the other hand, the other conditions listed have a more direct impact on heating effectiveness. An engine low on coolant would reduce the amount of heated fluid passing through the heater core, leading to decreased output. A stuck open thermostat prevents the engine from reaching optimal operating temperature, which also reduces heated coolant flow. A restricted heater control valve can block coolant flow entirely to the heater core, severely diminishing heating performance. Thus, the disengaged clutch-type radiator fan does not directly contribute to low heater output, making it the correct choice in this context.