Diagnostics and Troubleshooting
UNT-SVX041D-EN 81
Table 42. Diagnostics generated by UC400-B controller
Diagnostic Probable Cause Consequences Diagnostic Type
Filter change required
Fan run hours exceed the time set to
indicate filter change.
•Fan Unaffected
•Valves Unaffected
•Electric heat Unaffected
Informational
Condensate overflow The drain pan is full of water.
•Fan OFF
•Valves Closed
• Outdoor air damper Closed
•Electric heat OFF
Manual
Low coil temp detection
The leaving fluid temperature may be
close to freezing.
•Fan OFF
•Valves Open
• Outdoor air damper Closed
•Electric heat OFF
Smart reset/Manual
Low airflow supply fan
failure
The fan drive belt, contactor, or motor has
failed.
•Fan OFF
•Valves Closed
• Outdoor air damper Closed
•Electric heat OFF
Manual
Space temperature
failure
(a)
Invalid or missing value for zone
temperature.
• Discharge air temperature control runs
• Unit shuts OFF if both space temperature and
discharge air temperature fail
Automatic
Entering water temp
failure
Invalid or missing value for zone
temperature.
•Fan Unaffected (enabled)
•Valves Unaffected
• Outdoor air damper Unaffected
•Electric heat Unaffected
Automatic
Discharge air temp low
limit
Discharge air temperature has fallen
below the Discharge Air Temperature Low
Limit.
•Fan OFF
•Valves Open
• Outdoor air damper Closed
•Electric heat OFF
Smart reset/manual
Discharge air temp
failure(a)
Invalid or missing value for discharge air
temperature.
• Simplified zone control algorithm runs
• Unit shuts OFF if zone temperature fails
Automatic
Outdoor air temp failure
Invalid or missing value for outdoor air
temperature.
•Fan Unaffected
•Valved Unaffected
• Outdoor air damper Minimum
Position
•Electric heat unaffected
Automatic
Humidity input failure
Invalid or missing value for relative
humidity.
•Fan Unaffected
•Valves Unaffected
• Outdoor air damper Unaffected
•Electric heat Unaffected
Automatic
CO
2
sensor failure Invalid or missing value for CO
2
.
•Fan Unaffected
•Valves Unaffected
• Outdoor air damper Unaffected
•Electric heat Unaffected
Informational
Generic AIP failure
Invalid or missing value for generic
analog input.
•Fan Unaffected
•Valves Unaffected
• Outdoor air damper Unaffected
•Electric heat Unaffected
Informational
Local fan mode failure
Invalid or missing fan-speed switch
(reverts to default fan speed).
•Fan Unaffected
•Valves Unaffected
• Outdoor air damper Unaffected
•Electric heat Unaffected
Automatic
Local setpoint failure
Invalid or missing value for zone
temperature setpoint (reverts to default
setpoint).
•Fan Unaffected
•Valves Unaffected
• Outdoor air damper Unaffected
•Electric heat Unaffected
Automatic
(a) For detailed information about zone temperature control methods, refer to “Zone Temperature Control,” p. 65.
Fans with UC400-B Controller
Table 43. Fan does not energize
Probable Cause Explanation
Unit wiring
The wiring between the controller outputs and the fan relays and contacts must be present and correct for normal
fan operation. Refer to applicable wiring diagram.
Failed end device The fan motor and relay must be checked to ensure proper operation.
Normal operation
The fan will turn OFF when:
• The controller receives a communicated off signal
• The fan-speed switch is set to OFF if no communicated value is present
• Specific diagnostics are generated
• The default fan speed is set to OFF and the fan is operating in the Auto mode.
If the controller is in unoccupied mode, the fan cycles between OFF and the highest fan speed.
No power to
the controller
If the controller does not have power, the unit fan does not operate. For the controller to operate normally, it must
have an input voltage of 24 Vac. If the Marquee/Power LED is OFF continuously, the controller does not have
sufficient power or has failed.
Diagnostic present Several diagnostics affect fan operation. For detailed information about these diagnostics, refer to Table 42, p. 81.
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