Trane ACONT900AC43UA Service Manual page 300

Solving Err 91 Comm Errors
Detail 2: Connect a digital DC voltmeter to the RX
wire removed from E9 plug and the B/C low voltage
terminal board.
Detail 3: Re-power the gas furnace.
Is the DC voltage between the wire removed from
the gas furnace IFC plug E9 terminal RX and the
low voltage terminal B/C going up and down
between 0.145 volts DC and 0.208 volts DC? (See
Figure 9.)
Figure 9
Yes: Replace the gas furnace IFC.
No: Go to Step 6. Do NOT reconnect the RX wire.
Step 6
De-power the gas furnace. Connect the DC volt as
shown and then re-power the gas furnace.
R - B 6
Is there DC voltage between the inducer motor
drive board terminal TX and GND? (See Figure
10.) This DC voltage should be going up and down
between 0.145 volts DC and 0.208 volts DC.
Figure 10
Yes: Repair or replace the low voltage harness.
No: Replace the inducer motor drive board and
reconnect the wire that was removed from the gas
furnace IFC plug E9.
Step 7
Is the 12 VDC at the gas furnace IFC plug E9
measured between terminals 12V and GND? (See
Figure 11.)
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.