Trane / American Standard Error 91: Communication Error (Meaning, Causes, Fixes)

What the code means

Trane's communicating systems pass data between the comfort control (thermostat), the indoor unit and the outdoor unit over a low-voltage bus, and inside some furnaces between the main control board, the blower motor and the inducer drive board. Trane's System Alert Guide (August 2020) and its ComfortLink II alert code table list a family of 91 alarms, all rated Critical. ERR.091.00 Blower Communication Loss: loss of communication between the serial communicating blower motor and the control board (CLII / AccuLink units only); on 3-stage and modulating furnaces a corrupted signal between the furnace control board and the inducer drive board can also cause it. ERR.091.01 Inducer Communication Loss: loss of communication between the furnace control board and the draft inducer motor control board. ERR.091.02 System Communication Error: loss of heat/cool demand; the comfort control sends demand every minute, and the fault is reported after 3 missed messages. ERR.091.03 No System Clock: the system clock, set by the Bit Master on the indoor control board (air handler or furnace) or relay panel, is not reporting. ERR.091.05 Communication Mis-Wire Error: the R and D wires are reversed at the comfort control or indoor unit. ERR.091.06 Communication Error, Low Bus Voltage: bus voltage stuck low. ERR.091.99: the HVAC communication bus did not initialize correctly, usually after a thermostat reboot that didn't finish starting up.

Likely causes

• 91.00: no line voltage to the blower motor or inducer motor control board (115 VAC on a furnace, 230 VAC on an air handler), or a bad low-voltage connection between the control board and the blower motor. Trane also lists a failed control board or blower motor, decided by a 24-volt go/no-go test.
• 91.00 on 3-stage and modulating furnaces: the inducer drive board corrupting the signal. Trane's test is to unplug the 4-pin connector at E9 on the furnace board, cycle power and see whether the blower runs.
• 91.01: a bad connection or harness between the furnace control board and the draft inducer motor control board, or a failed inducer drive board. On a call for heat the green COM LED on the inducer board should flash.
• 91.02: the data (D) line to the thermostat broken or disconnected, D and the common (B) line reversed at the outdoor unit, or the D line shorted to ground on the way to the outdoor unit. A communicating air cleaner on the D wire can also be the cause.
• 91.03: the thermostat isn't communicating with the indoor unit or relay panel. Trane's reference is about 12 VDC for good communication and 16 VDC for lost communication.
• 91.05: R and D wires swapped at the thermostat or indoor unit.
• 91.06: a communicating air cleaner with a faulty control board, or D and B wires reversed at the thermostat, outdoor unit or indoor unit.
• 91.99: a thermostat reboot that didn't fully initialize its internal components.

Safe DIY checks

1. Write down the full alert, including the digits after the dot (91.00, 91.02 and so on). The fix depends entirely on which one it is.
2. For 91.99, do what Trane says: power the thermostat down for 5 seconds, then power it back up. Turning off the breaker for the indoor unit for about a minute and then back on is a reasonable next step for the other 91 codes too.
3. If you have a communicating air cleaner (one wired into the thermostat's communication wires), note it for the technician. Trane's own test for 91.02 and 91.06 is to disconnect the air cleaner or its D data wire and see whether the alert clears.
4. If someone recently replaced the thermostat or rewired anything, mention it. Swapped R and D wires (91.05) or D and B wires (91.06) are installation mistakes Trane names directly.
5. Walk the visible thermostat wire run to the outdoor unit and look for cut, chewed or crushed cable, which can ground the data line (91.02). Don't splice it yourself.
6. Leave the furnace and outdoor unit panels closed. The blower and inducer checks in 91.00 and 91.01 involve line voltage and live board testing.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 180.02 Trane HVAC System 180.02
• 67.06 Trane HVAC System 67.06
• 106.00 Trane HVAC System 106.00
• 126.00 Trane HVAC System 126.00
• 156 Trane HVAC System 156

All Trane codes →

Sources

1. Trane System Alert Guide (Aug 2020), pp. 18-19: ERR.091.00 Blower Communication Loss, 091.01 Inducer Communication Loss, 091.02 System Communication Error, 091.03 No System Clock, 091.05 Communication Mis-Wire Error, 091.06 Low Bus Voltage, 091.99 bus did not initialize (distributor-hosted copy)
2. Trane ComfortLink II alert codes table: CL2.091.00-091.06, all Critical (Blw Comm Err, Ind Comm Err, Sys Comm Err, No Sys Clk, Bus Mis-wire, Bus Stuck Lo) (distributor-hosted copy)