T500 Series Programmable Thermostats Product/Technical Bulletin
21
T
roubleshooting
Table 8: Troubleshooting
Symptom
Possible Cause
Corrective Action
No display/faint display ...
Supply voltage incorrect
Use a voltmeter to check the voltage between the 24V and
24V(c) terminals. Voltage should be between 20-30 VAC.
If voltage is less than 20 VAC, disconnect the thermostat
and check the voltage between 24V and the other system
wires; see possible causes below. If voltage is greater than
30 VAC, troubleshoot the power source and replace the
thermostat.
System transformer weak or
overloaded
Check and/or replace with a suitable 24V transformer.
Thermostat damaged
because system voltage
was greater than 30 VAC
Replace with new thermostat and ensure new thermostat is
isolated from the system using suitable relays and a
transformer of the proper rating.
Keyboard inoperative ...
Keyboard locked
Switch the keyboard DIP switch to the unlocked position.
Thermostat will not call for
heat ...
Compressor delay still in
progress
Wait - equipment short cycle protection in progress.
Thermostat setpoint is
satisfied
Raise the heating setpoint using the
∧∧∧∧
button.
Thermostat will not call for
cooling ...
Compressor delay still in
progress
Wait - equipment short cycle protection in progress.
Thermostat setpoint is
satisfied
Lower the cooling setpoint using the
∨∨∨∨
button.
Fan does not turn on ...
Fan failure
Place a jumper between terminals R and G. Fan should
come on. If it does not, troubleshoot the fan system. If fan
does come on, replace the thermostat.
AC appears on the LCD ...
20-30 VAC is absent from
24V and 24V(c)
Using a voltmeter, measure voltage between the 24V and
24V(c) terminals. If the reading is less than 20 VAC, check
system transformer. If the voltage is between 20-30 VAC,
replace the thermostat.
LCD shows missing or
extra segments ...
LCD failure
Replace the unit.
