Carrier Outdoor Unit Communication Fault (Code 179) – Easy Diagnosis

The outdoor and indoor units of your HVAC system have to be constantly communicating, in order to keep your house at a comfortable temperature. If there is a communication fault, the system won’t be able to operate effectively.

A Carrier outdoor unit communication fault is designated by a 179 code. An issue with the wiring anywhere in the system can cause such an error. To find out the root cause of the problem, you would have to check the communication wires and the float switch.

How Do You Read a Carrier Furnace Error Code?

Carrier furnaces have lights that will start blinking if an error ever occurs. Each fault code is a two-digit number.

Carrier furnace code light

To find out the digits, pay attention to the short and long flashes (the short flashes determine the first number and the long ones – the second). For example, one short flash and three long flashes correspond to error code 1-3 – ‘limit or flame roll-out switch lockout’.

Read: Why Is Carrier Thermostat Blinking Cool ON? – Troubleshooting Guide

What Is a Code 24 on a Carrier Furnace?

Fault code 24 indicates that the secondary voltage fuse is open. You would have to replace either the fuse or the control board.

secondary voltage fuse

Carrier Infinity System Malfunction Sensor Error

It looks like the sensor had gone bad and it is no longer recognized by the circuit board. To fix the issue, you would have to install a new sensor.

Carrier System Fault 126-High Heat Cycle Active

Such a fault code can appear if the system is overheating. Make sure that the supply and return grills are not blocked, that the air filter is not clogged, and that the ducts are clean.

If the error does not go away, then there might be an issue with the gas valve and the manifold pressure. In such a case, it is better to call a professional. 

Read: Carrier Furnace Error Codes – Troubleshooting Guide

What Is a Code 13 on a Carrier Furnace?

Error code 13 means ‘limit circuit lockout’ – the flame or roll-out switch has been open for over 3 minutes. Check if the system has proper airflow and make sure that the actual switch is not faulty. 

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Furnace with access panel removed

Carrier Outdoor Unit Communication Fault

A loose or damaged wire anywhere in the system can cause an ‘outdoor unit communication’ fault. Basically, even when the system displays that the connection has been lost with the outdoor unit, the actual loss might have happened either inside or outside.

You would have to change the wiring for the communication wire.

Carrier Fault Code 179 – Outdoor Unit Communication

First things first, make sure that you have power to the outdoor unit. If this part of the system is not receiving any power, then it will not register. 

Also, do check the outdoor circuit board – there shouldn’t be any blown fuses or tripped breakers.

If the code appears on your Carrier Infinity thermostat, then you should:

  1. Make sure that all the wires are not loose and that they are connected to the right terminals.
  2. If there are no issues with the wiring, then check the float switch (it’s located in the inside unit’s drain line). The outdoor unit will stop communicating if the float switch had tripped.
float switch

Carrier Infinity Indoor Unit Communication Fault 178

An indoor unit communication fault does not necessarily mean that the problem is with the indoor unit. You might have to check the entire system.

  1. Check the wiring to the ABCD connector (those are the communication wires) on the outside unit.
  2. Each wire should be connected to the right terminal and the connections have to be secure.
  3. If you feel comfortable working with electricity, then you can check the 24V between the D and C terminal connector.

Read: Why Furnace Is Making A Clicking Noise – Troubleshooting Guide

What Does System Malfunction Mean?

On a Carrier unit, ‘system malfunction’ can mean a wide range of things. You can press and hold the right-side button for a few seconds – the thermostat will display any current errors that the system is experiencing, this will help you get a better understanding of the problem.

If you need the heating and cooling unit to start operating, then you can dismiss the ‘system malfunction’ message. Simply press the right-side button and the message will appear where the date/time should be.

You can also try resetting the system. 

  • Turn off the power to the unit at the circuit breaker. 
  • Take out the batteries from the thermostat.
  • Put them back in backward for about 30 seconds. 
  • Replace the batteries in the correct position and turn the system back on.
Old thermostat batteries can cause Carrier outdoor unit communication fault

Do bear in mind that calling an HVAC expert that would take care of the problem is the best possible decision. 

How Do I Know If My Carrier Infinity System Malfunctions?

You can enter the diagnostic mode on your thermostat:

  1. Open the flip-down door and press and hold the ADVANCED button for around 10 seconds.
  2. Once a menu appears on the screen, scroll down to highlight CHECKOUT.
  3. Press SELECT – the device will check every component in the system.
  4. Use the scroll button to highlight SERVICE INFO, and select this option. You will get to see the error codes (if there are any) and the corresponding component.

Read: Best Furnace Flame Sensors

How Do You Reset the Code on a Carrier Furnace?

All stored status codes are going to be erased as soon as the power to the system is interrupted. So, you can simply reset the unit.

Is There a Reset Button on a Carrier Air Conditioner?

The reset button is usually located right on the control panel. The button can be quite small, but you should be able to find it thanks to a special label.

At times, you might have to search beneath the digital display to locate the button.

Where Is the Reset Button on a Carrier Furnace?

You’ll find the red reset button right in the middle of the flame roll-out switch. This switch is usually located on the top or the side of the burner compartment. See video below…

How Do I Reset My Carrier Controller?

  1. Press and hold the LEFT button until the ‘accessory status’ screen appears.
  2. Use the SCROLL button to choose the accessory that you would like to reset.
  3. Press the RIGHT button and the chosen accessory will get reset.
  4. To return to the home display, press the LEFT button.

How Do You Reset Carrier Infinity Heat Pump?

Sometimes, in order to address a Carrier outdoor unit communication fault, you must reset the heat pump.

  1. Turn off the heat pump at the breaker.
  2. Make sure that the fuse has not blown. Replace it, if necessary.
  3. Check the UI settings – if the settings are inappropriate, the compressor might not engage. 
  4. Reset the circuit breaker.

How Do You Override Carrier Infinity Thermostat?

You can temporarily override the scheduled cooling temperature settings and then return to the scheduled program

Where Is the Fuse Located on a Gas Furnace?

The majority of new furnaces, including gas units, have a fuse as the system requires an electrical connection to operate. Fuses protect the unit from a potential overload or short.

The exact location of the fuse will depend on the model of your furnace. Usually, the element can be found in the furnace housing unit.

Open the furnace access door – there, you will find the circuit board and, most likely, the fuse. These elements are often small, colored, and made out of plastic. 

Read: No Power To The Home Temperature Thermostat – Troubleshooting Guide

Where Is the Secondary Fuse on a Carrier Furnace?

Error code 24 on your Carrier furnace means that the secondary voltage fuse is open. All transformers are actually connected to the common secondary circuit via a secondary fuse (it is placed after the transformer’s output points of connection on the secondary side).

In most cases, you would have to remove the service door on the indoor unit and locate the circuit board. The fuse should be plugged right into the board (it might look like a car-type, purple fuse).

If you replace the fuse with a new one and it blows as well, then your system has a short.


Now that you know what a Carrier outdoor unit communication fault code means, you can address it if it ever pops up in your system. Always be sure to check the user manual or specs of your particular unit, as each model can vary.