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Indoor Coil Thermistor
The indoor coil thermistor is located on the left
side of the indoor coil. This thermistor measures
the indoor coil temperature.
Indoor Coil Thermistor Diagnosing
If an indoor coil thermistor were to fail, the
resistance heater and fan will continue to operate,
but the compressor will not come on for cooling or
heat pump.
Check for Fault Codes
Check fault codes by pressing and holding MODE
and then press AUX. Verify whether fault 4
(Inside Thermistor Fault) code is active. Record
any other faults and clear the codes by pressing
AUX.
Verify Indoor Coil Thermistor Operation
Enter Service Mode by pressing and holding +
and - buttons. On entry, the display lights all
LED's. Press MODE until S2 is shown in the
display. Pressing + displays the indoor coil
temperature. AUX key exits Service Mode.
Verify Indoor Coil Thermistor Resistance
Value
Indoor coil thermistor resistance values can be
checked from the main control board at the J502
connector.
• Pin 3 (black) to pin 4 (black): Use Indoor Coil
Thermistor Resistance chart below to verify
resistance to actual temperature.
Indoor Coil Thermistor
Resistance (ohms)
°F
10
30
32
50
70
90
Air
28,030
15,340
14,570
8,850
5,300
3,280
Indoor Coil Thermistor Removal
1. Remove junction box cover, main control
board cover, top cover panel and left side
panel.
Indoor Coil
Thermistor
2. Clip zip tie and pull the indoor coil thermistor
out of its holder. NOTE: Remember to
reattach another zip tie when reinstalling the
new thermistor.
Zip Tie
– 29 –
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