A temperature control system (10) of an automatic film processor (12) includes developer and fixer recirculation paths (30, 40) having thermowell heaters (34, 44) and thermistors (35, 45), and a cooling loop (37) in the developer path (30) which passes in heat exchange relationship with water in a wash tank (23). The system (10) also has a blower (48), heater (49) and thermistor (52) in an air path of a dryer (24). Actual heating and cooling rates of heating and cooling cycles are determined based on temperature measurements by the thermistors (35, 45, 52). Heater (34, 44, 49) and cooling loop (37) operation is controlled by comparing measured temperatures with preestablished setpoint temperatures. Malfunctions of system (10) are identified by comparing actual rates with rates characteristic of normal operations. Measured temperature data is validated based on comparing measured temperature with temperature predictions calculated based on heat gain or loss relationships associated with particular heating or cooling cycles. Randomly occurring invalid data is disregarded for control and error diagnosis purposes.
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