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The thermostat is the most significant piece of equipment in our home. When it malfunctions, we become terrified thinking of the worst-case scenarios. Trane thermostats are a sigh of relief in this case.
With a few exceptions, they rarely fail and require repair. One of those exceptions includes the display abruptly showing error code 91.
But what is trane error code 91?
Trane error code 91 is a failure of communication between the indoor and outdoor units. Sometimes it’s a loss of cool and heat demand which causes the communication loss. And other times it’s a malfunctioning system clock. The furnace and the inducer motor also lose connection, resulting in error 91.
But are those the only reasons for communication errors? Or are there further reasons? And how do you solve these issues? We have the answers right here.
So let’s not waste anymore time and get right at it!
What Is Trane Error Code 91?
The Trane comfort link ii is an excellent thermostat. It does a lot more than just control your home’s temperature. It also functions as a zoning control and air quality management tool.
This thermostat will assist you in optimizing your home’s energy management year after year. However, the thermostat can occasionally malfunction.
One such case is the error code is 91. Error Code 91 indicates a communication failure or error. This tends to happen between the equipment and the thermostat. This is an error that occurs among a lot of users.
This can occur due to power surges, outages, or other power-related problems.
What’s The Reason Behind Error Code 91?
A communication issue is the cause of the trane error code 91. However, it might be a blower motor, control issue, or even an inducer motor issue.
Behind these problems, power surges are the most common reason. If power surges are common in your area(like South Florida), you may experience this error as a result.
Your display will only show Trane Error 91 in all of the communication error instances. But the indoor unit, which is usually installed in your attic will specify what the problem is.
Your unit will also display various LEDs. The fault code which flashes with 91 error codes will be 0, 1, 3, or 10.
Let’s have a look at these different communication errors and why they happen-
Case 1: System Clock Not Reporting
The User Interface will be NO SYS CLK when showing Err 91. The furnace Fault LED will have 10 flashes. But the Air Handler Fault LED will have only 3 flashes.
This failure will turn off the Bit master LED. For that, the system clock won’t be reporting at all. All of the internal parts are synchronized by the system clock.
The system clock generates a high-frequency signal in order to synchronize them. Electrostatic discharge or voltage spikes can easily disrupt this signal. And cause errors in the communication.
Case 2: Loss of Cool & Heat Demand
The User interface shows SYS COMM ERR with 3 flashes from Air Handler LED. Furnace Fault will have 10 flashes in this case as well.
Comfort Control generates heat and cool demand per minute. But if the demand message is not received then this error is usually displayed on the interface.
However, this doesn’t occur until the demand message has failed three times. A couple of different malfunctioning wiring can cause this while showing error code 91-
- The data (D) line that is connected to comfort control might be disconnected or broken. You can check the voltage in between the lines.
In this case, it will be 0vDC between the D and B wires at comfort control. But the indoor unit will have 12 VDC.
- The wirings of the data (D) line and common (B) line are usually connected at outdoor units. These wires can easily be reversed.
If that is the case the LED of the Bit Master will be dim or off. Sometimes the LED also flickers.
The voltage you can read when the wires (D & B) are connected will be 0vDC. But if disconnected it will be 12 VDC.
- The data (D) line can be grounded while going to the outdoor unit connection. The flickering or dim Bit Master LED will be the main indication of this case.
In connected and detached modes, the voltage reading between wires will be the same as before.
Case 3: Communication Loss Between the Furnace and the Inducer Motor
This communication error happens because of a faulty inducer drive board. The inducer motor ensures that your furnace operates safely.
Usually, it takes some time for the furnace to work. However, in this case, without the motor running and no connection between them, it will turn off.
The wiring between the boards can also be the reason behind connection loss. The interface will show IND COMM ERR with 10 furnace Fault LED flashes continuously.
Case 4: Loss of Clock Signal From Indoor Unit
This error will have the outdoor unit fault LED flashing once. But the Indoor one won’t be reporting any flashes. The display will have No Alert.
The comfort control won’t have any heat or cool demand message. Basically, the whole system will have communication errors going through.
Case 5: Communication Loss Between the Blower Motor and Control Board
3 flashes will be shown on the Air Handler LED in this case. And the user interface will show BLW COMM ERR. The Furnace Fault LED will also show flashes. Exactly 10 flashes will be shown from this one.
Total loss of communication between the control board and blower motor. The inducer motor and furnace usually have corrupted signal or no signal at all in this case.
Case 6: Dead Communication Line
No Alert Display will be on the user interface. However, the Air handler will show no fault report. None of the information will be received as the line is dead.
How to Solve Trane Error Code 91?
Communications errors are usually because of something wrong with the control board or the motors. You can’t simply just reset the Trane thermostat system and the problem will be gone.
The wiring can be wrong or the boards are acting up. But before you do any troubleshooting you need to ground the furnace. In some cases, you might need to change something as small as the wirings. This wiring is a little tricky for most amateurs.
The control board or the furnace might need replacing. These are all quite tricky fixes. So, if you observe something that seems like the conditions listed above, it’s best to seek help.
But if you are changing the control boards, let us suggest some durable boards for you.
If you’re looking for the original control board, you can check White Rodgers Replacement for Trane Furnace Control. It’s really easy to install.
Both of these boards are recommended by professionals for durability and cost-effectiveness.
Frequently Asked Questions (FAQs)
How Long Does A Trane Air Conditioner Last On Average?
Trane air conditioners last on average for about 12-years. That’s based on its warranty. These air conditioners are estimated to last around two years longer than other brands. Your particular unit may need replacement sooner or later than 12 years.
What Causes A Thermostat To Stop Working Properly?
Your thermostat not getting any power connection could make it stop working. A tripper circuit breaker, dead battery, or a blown fuse can also cause this. And if these are all working the issue might be somewhere else.
How Can I Tell If My Thermostat’s Battery Needs To Be Replaced?
Your battery will last 1-2 months after showing the “Low Battery” indicator. This appears on your display after the battery is drained. Instead of an on-display indication, some thermostats include a light.
End Words
That’s all we have to say about trane error code 91. If this code appears on your thermostat display, don’t panic and try to follow our instructions.
Please do not hesitate to seek assistance if trane error 91 appears on the screen. Trane provides amazing customer service. But you can also call a local electrician to help you with this.
Good Luck!
HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.