R - B 9
Solving Err 91 Comm Errors
Figure 16
Detail 2: Connect a digital DC voltmeter to the RX
wire removed from the low voltage plug and the B/C
terminal at the low voltage terminal board.
Detail 3: Re-power the gas furnace.
Is there DC voltage between the wire removed
from the gas furnace IFC or the air handler control
board plug low voltage terminal RX and the low
voltage terminal B/C going up and down between
0.152 volts DC and 0.174 volts DC? (See Figures
17a & 17b.) The voltage meter will stop reading at
times and then start reading again.
Figure 17a
Figure 17b
Yes: Replace the gas furnace IFC or the air handler
control board.
No: Go to Step 6. Do NOT reconnect the RX wire.
Step 6
De-power the gas furnace or the air handler. Connect
the DC voltmeter as shown and then re-power the
gas furnace or the air handler.
Is there DC voltage between the indoor blower
terminals TX and GND? (See Figure 18.)
Figure 18
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