R - B 13
Solving Err 91 Comm Errors
Does the comfort sensor and furnace IFC or air
handler now communicate?
Yes: Err 91 is being caused by the thermostat wire
or its routing. Repair, reroute or replace the
thermostat wire.
No: Replace the comfort sensor.
Step 8
Does the furnace IFC (See Figure 26) or air
handler fault LED show a fault flash code?
Yes: Go to Step 5.
No: Go to Step 9.
Figure 26
Step 9
Does the outdoor unit Charge Assist™ Control
have 24 volts AC at plug J2 terminals R and B?
(See Figure 27.)
Note: When any of the Charge Assist™ LEDs are
on or flashing, this only indicates that some low
voltage AC is present. Check the 24 volts AC supply
with a volt ohm meter.
Yes: Go to Step 10.
No: 24 volts AC is supplied by the outdoor unit’s trans-
former. Repair or replace as needed.
Figure 27
Step 10
Does the outdoor unit Charge Assist™ Control
fault LED show a fault flash code of one flash?
Yes: Check the DC voltage at test plug J2 terminals
D and B. If this voltage is 16 to 17 volts DC, this
indicates that the D or B or both thermostat wires
coming from the furnace or air handler are open (See
Figure 28.) Repair or replace the thermostat wire.
No: Normal voltage at this point is 12.5 volts DC.
Go to Step 11.
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