Lennox XC25 – Unit Sensor Operations, Checkout and Status / Error Codes


Pins 5 and 6 (No Sensor)
There is no sensor located on positions 5 and 6 of the connector. A resistor installed between pins 5 and 6 on the cable harness provides continuity for this circuit. This resistor (location shown in figure 51) provides resistance value between 314669 and 3779 ohms . If the resistor becomes damaged or lost, the outdoor control generates a false error code indicating the coil temperature sensor is out of range or faulty (see table 15 on page 68). If resistor is damaged or missing, order re­placement harness assembly (101334-05).

Ambient Temperature Sensor (RT13)
Ambient temperatures, as read by the ambient temperature sensor (shown in figure 51) which are below -35°F (-37°C) or above 120°F (48°C) trigger a fault condition. If the ambient sensor is open, shorted, or out of the temperature range of the sensor, the control does not perform demand defrost operation. The control reverts to time/temperature defrost operation and displays the appropriate alert code (see table 15 on page 68). Heating and cooling operation is allowed in this fault condition.

Liquid Line Temperature Sensor (RT36)
Liquid line temperatures below -40°F (-40°C) or above 140°F (60°C) trigger a fault condition and result in the appropriate alarm code (see table15 on page 68). The outdoor control uses the liquid line sensor to change the fan speed, allowing cooling operation at lowered ambient temperatures.

Sensors connect to the outdoor control through a field?replaceable harness assembly that plugs into the outdoor control. Through the sensors, the control detects outdoor ambient, coil and liquid temperature fault conditions. As the detected tem­perature changes, the resistance across the sensor changes. Check sensor operation by reading ohms across pins shown in figure 50.

NOTE – When checking the ohms across a sensor, be aware that a sensor showing a resistance value that is not within the range shown in figure 50, may be performing as designed. However, if a shorted or open circuit is detected, then the sensor may be faulty and the sensor harness will need to be replaced.



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