R - B 5
Solving Err 91 Comm Errors
Yes: Go to Step 5.
No: Go to Step 4.
Step 4
Set the thermostat to call for heat.
Is there DC voltage between the gas furnace IFC
plug E9 terminals TX and GND? (See Figure 7.)
This DC voltage should be going up and down
between 0.2 volts DC and 1.7 volts DC*.
*Note: Voltage levels may vary depending upon volt
OHM meter type.
Yes: Repair or replace the low voltage harness going
from the gas furnace IFC plug E9 to the inducer motor
drive board.
No: Replace the gas furnace IFC.
Figure 6
Figure 7
Step 5
De-power the gas furnace. At the gas furnace IFC<
remove the wire from the terminal RX at the E9 plug
going to the inducer motor drive.
Detail 1: Push down on the wire connector latch clip
on the side of the plug and pull the RX wire from the
E9 plug. Plug the E9 back into the IFC (See Figure
8.)
Figure 8
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