R - B 14
Solving Err 91 Comm Errors
Figure 28
Step 11
Connect one wire at a time back onto the D terminal
of the furnace IFC or air handler terminal board.
These are the wires removed in Step 2.
Does the BM LED stay on solid and bright when
each wire is reconnected one at a time to the D
terminal?
Yes: Check the control system for proper operation.
No: When a thermostat wire is reconnected to the D
terminal and the BM LED goes out, goes dim, or starts
flashing, this indicates that this is the circuit which is
causing the ERR or fault. Remove this thermostat
wire and determine where this thermostat wire goes,
then go to Step 12.
Step 12
Go to the component that is being fed by the
thermostat wire that was determined to be creating
the ERR or fault in Step 11 and disconnect the
thermostat wire going to that component. Now go
back to the furnace or air handler and reconnect the
thermostat wire that was determined in Step 11 to
be the circuit causing the problem.
Does the BM now stay on solid and bright?
Yes: This proves the thermostat wire going to the
component is good.
No: This ERR or fault is being caused by the
thermostat wire. Repair as needed or replace the
thermostat wire.
Communicating system voltage readings
Note: These measurements to be made at
components LVTB, thermostat sub base or field
wiring connections at outdoor unit. Line voltage
applied to the unit.
Normal operating voltage
R = Power = 24 volts AC
D = Data = 12 volts DC
B = Common (B terminal is common for AC and DC
voltage readings)
Furnace, air handler, comfort control, TAM or outdoor
unit:
R to B = 24 volts AC
D to B = 12 volts DC
If you read R to D you will read 12 volts DC or 24
volts AC, depending on how you set up your
voltmeter.
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