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DEMAND DEFROST CONTROL (A108) DIAGNOSTIC LEDS
The state (Off, On, Flashing) of two LEDs on the demand defrost control (DS1 [Red] and DS2 [Green]) indicate diagnostics
conditions that are described as follows.
Table 9. Demand Defrost Control (A108) Diagnostic LEDs
DS2
Green
DS1
Red
Condition/Code
Possible Cause(s)
Solution
OFF
OFF
Power problem
No power (24V) to demand defrost
control terminals R and C or demand
defrost control failure.
1
Check control transformer power (24V).
2
If power is available to demand defrost control and
LED(s) do not light, replace demand defrost control.
Simultaneous
SLOW Flash
Normal operation
Unit operating normally or in standby
mode.
None required.
Alternating SLOW
Flash
5−minute anti−short cycle delay Initial power up, safety trip, end of room
thermostat demand.
None required (Jumper TEST pins to override)
Simultaneous
FAST Flash
Ambient Sensor Problem
Sensor being detected open or shorted or out of temperature range. Demand defrost control will
revert to time/temperature defrost operation. (System will still heat or cool).
Alternating
FAST Flash
Coil Sensor Problem
Sensor being detected open or shorted or out of temperature range. Demand defrost control will
not perform demand or time/temperature defrost operation. (System will still heat or cool).
ON
ON
Demand Defrost Control
Failure
Indicates that demand defrost control has internal component failure. Cycle 24VAC power to de-
mand defrost control. If code does not clear, replace demand defrost control.
Table 10. Demand Defrost Control (A108) Diagnostic Fault and Lockout Codes
DS2
Green
DS1
Red
Condition/Code
Possible Cause(s)
Solution
(Each fault adds 1 strike to that code’s counter; 5 strikes per code = LOCKOUT)
OFF
SLOW
Flash
Low Pressure Fault
1
Restricted air flow over indoor or
outdoor coil.
2
Improper refrigerant charge in
system.
3
Improper metering device installed or
incorrect operation of metering
device.
4
Incorrect or improper sensor location
or connection to system.
1
Remove any blockages or restrictions from coils
and/or fans. Check indoor and outdoor fan motor for
proper current draws.
2
Check system charge using approach and subcooling
temperatures.
3
Check system operating pressures and compare to
unit charging charts.
4
Make sure all pressure switches and sensors have
secure connections to system to prevent refrigerant
leaks or errors in pressure and temperature
measurements.
OFF
ON
Low Pressure Lockout
SLOW
Flash
OFF
High Pressure Fault
ON
OFF
High Pressure Lockout
SLOW
Flash
ON
Discharge Line Temperature
Fault
This code detects shorted sensor or high discharge temperatures. If the discharge line temperature
exceeds a temperature of 285ºF (140ºC) during compressor operation, the demand defrost control
will de−energize the compressor contactor output (and the defrost output if active). The compressor
will remain off until the discharge temperature has dropped below 225ºF (107ºC).
FAST
Flash
ON
Discharge Line Temperature
Lockout
OFF
Fast
Flash
Discharge Sensor Fault
The demand defrost control detects open sensor or out of temperature sensor range. This fault is
detected by allowing the unit to run for 90 seconds before checking sensor resistance. If the sensor
resistance is not within range after 90 seconds, the demand defrost control will count one fault.
After 5 faults, the demand defrost control will lockout.
Fast
Flash
OFF
Discharge Sensor Lockout
