R - B 12
Solving Err 91 Comm Errors
If BLW COMM ERR is displayed (Figure 22), go to the
Troubleshooting BLW COMM ERR section.
Figure 22
BLW COMM ERR
If NO ALERTS is displayed (See Figure 23), go to
Step 9.
Figure 23
NO ALERTS
Step 6
Check the thermostat wire going to the comfort
sensor. The B thermostat wires going from B terminal
at the furnace IFC or air handler terminal board must
go to the B terminal on the comfort sensor. If the B
and R wires are crossed, the comfort sensor will have
a display but it will not communicate and Err 91 will
be displayed. Check for 10 to 12 volts DC at the
thermostat sub base terminals D and B. This voltage
comes from the furnace IFC or air handler control
board (See Figure 24.)
Is the comfort sensor wired correctly and are 10
to 12 volts DC present?
Yes: Go to Step 7.
No: Repair or replace the thermostat wire.
Figure 24
Step 7
Turn power off to the furnace or air handler. Disconnect
the comfort sensor and sub base and reconnect it to
the furnace IFC or air handler terminal board with a
short piece of thermostat wire. Disconnect all other
wires connected to the D terminal at the furnace IFC
or air handler terminal board. Turn power back on to
the furnace or air handler (See Figure 25.)
Figure 25
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