R - B 7
Solving Err 91 Comm Errors
Figure 11
Troubleshooting BLW COMM ERR
A gas furnace fault LED will be flashing a ten flash
code.
An air handler fault LED will be flashing a three flash
code.
Step 1
Power is on during this step. Is there 12 VDC at the
indoor blower low voltage plug terminal 12V and
GND? (See Figure 12.)
Figure 12
Yes: Go to Step 2.
No: Go to Step 7.
Note: The colors of the low voltage V.S. blower motor
wires are:
Gas furnace uses a blue wire and a black wire.
Air handler uses a blue wire and a brown wire.
Step 2
Is there 120 VAC at the indoor blower high voltage
plug terminals? (See Figure 13.)
Figure 13
Yes: Go to Step 3.
No: Repair as needed and retest.
Note:
1. The correct line voltage is:
Gas furnace-120 volts AC
Air handler-220 volts AC
2. The colors of the line voltage wires are:
Gas furnace has a black and a white wire
Air handler has a black and a red wire
Step 3
Set the Comfort Control thermostat to call for blower
operation.
Is there DC voltage between the indoor blower
motor low voltage plug terminal RX and GND?
(See Figure 14.)
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