Apparatus for and method of long-term control of the temperature within a refrigerated space, such as the cargo compartment of a refrigeration truck. The technique employs sensors which monitor both discharge and return air. A control algorithm uses the multiple sensors to control modes of a refrigeration unit capable of supplying cooled or heated air. This causes the temperature within the refrigerated space to remain within a narrow range about a target temperature. The target is chosen to provide maximum safety for the cargo. If the target temperature is too high, the cargo is needlessly exposed to spoilage. If the target temperature is too low, the cargo is subject to the risk of damage from freezing.