A broken blend door cable will make HVAC output temperature uncontrollable. The blend door controls the temperature of the air by routing the air through or around the heater core.

Cold temperature, blend door position.

When the control is set to cold, the blend door blocks the air from going past the heater core.

Hot temperature, blend door position.

When the control is set for hot, the blend door routes all of the air past the heater core.

Warm temperature, blend door position.

When the control is set between hot and cold, the blend mixes the air by allowing some of the air to go past the heater core and some to by-pass it.

Components that do not affect heater temperature control.

A defective compressor clutch will not affect heater temperature control.

A clogged orifice tube will not affect heater temperature control.

An inoperative blower motor has no effect on temperature control.

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