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Carrier Thermostat Intermittent ProblemCarrier Thermostat Intermittent Problem - I have a Carrier thermostat that recently has started to oscillate between off, cool and heat, seemingly randomly. We set the temps for cooling and heating, time, day, and all of those programmed pieces of data remain, but the t-stat just randomly switches between heat, cool and off (doesn’t ever go blank, it just displays “off”).

The switching between each setting might happen every second, or there might be a minute or so between the setting changes.

Wondering where I should begin looking to diagnose the problem.

Thanks in advance! Jeff

Carrier Thermostat Intermittent Problem Troubleshooting

Troubleshooting Carrier Thermostat

The wires here were stripped to far up the insulation jacket. There should be little to no exposure of copper.

Troubleshooting Carrier Thermostat Guide - My gut reaction is to say its the thermostat but I could be wrong. What would I do? I would carefully go over the control wiring running from the thermostat to the air handler. How would I do this? I would disconnect it from the air handler and the thermostat (turn off power first). Then, with the help from a friend or helper, I would ring out the wires.

When I say that I mean to say I would Ohm them out to make sure they all have continuity. If the wires have continuity while they are separated then you have a problem with the wire. Since your problem is intermittent this method is not foolproof. Has there been any work done in the house where something has penetrated the walls where the wire runs?

Solving the Problem Through the Process of Elimination

The reason I say this is because the wiring could be the problem. Since the problem is intermittent then this will be a difficult issue to solve unless it does it while you are testing the wiring. Therefore, the wires would all ring out good and you put everything back together and it will work fine for a week and then do it again. However, if some work was done that damaged the wire inside the walls it could cause this problem.

Steps for Troubleshooting the Intermittent Problem

  1. Check the Wiring
  2. Ring it Out (Ohm it)
  3. Temporary Replacement of Thermostat
  4. Check the control board and control power for problems

Details for Carrier Thermostat Troubleshooting

Check the Wiring

1Check the wiring. A careful, methodical inspection of the wiring and connections. Something could have pinched the wire bundle or fell on it from inside the wall and damaged it. Thermostat wire is not protected inside the wall (not ran in conduit). Furthermore, it’s also possible it is old and insulation on the wire has decayed allowing the wires inside the bundle to touch.

Additionally, thermostat wires improperly prepared can touch just behind the thermostat. The spliced bared copper should only have approximately 1/4 inch for the termination. The rest should be insulation. If the insulation is nicked and touching it could cause the problem you are having. Furthermore, this is at both ends.

Ring Out the Wires

Troubleshooting Carrier Thermostat2Since you can’t see the wiring behind the walls to check for damage ring it out. Use an Ohmmeter and a friend. A good form of communication will help also. That is because where the thermostat is located and where the air handler/furnace is located is usually remote. You’ll need to communicate to the friend such as: Okay, touch the red and yellow wires together. And touch the red and green wires together. So on and so forth until you’ve tested all possible combinations.

If you find a problem with the thermostat wiring it’s time to run new wire. Running new thermostat wire can be a challenge even for experts so be prepared. Call an HVAC professional if you are uncomfortable doing it.

Temporary Thermostat

3Replace the thermostat with a temporary thermostat and see if the problem continues. I know you likely don’t have a spare thermostat laying around like I do but this is troubleshooting and the spare would help me eliminate one component. A component that is the possible problem.

Keep the old Carrier thermostat since, at this point, you are unsure this is the absolute problem. Of course, not everyone has a spare thermostat laying around like I do. Therefore, you have a choice to make. Buy a new thermostat that you want or just buy the cheapest thermostat you can find. There is a risk buying a thermostat with all the bells and whistles because at this point the cause is undetermined. I have an Ecobee 4 Smart Thermostat and love it.

Air Handler or Furnace Control Board and Control Power

Ecobee Smart WiFi Thermostat

Ecobee Smart WiFi Thermostat

4After eliminating everything else and you continue to have the problem then the problem is in the air handler. This is an area where it is probably a good idea to call an HVAC professional. Most likely, a contractor that specializes in whatever equipment you have installed. I am assuming it is Carrier equipment but it could be another brand name. You could check the electrical connections on the control board and at the transformer. Make sure they are tight. Additionally, check where the wire was spliced (as you did above near the thermostat).

If you did the above steps and procedures explain to them what you did. The transformer should read at least 24 volts. I often see them reading higher up to 30 volts. Anything over that and I would be weary. A good visual inspection of the control board is also necessary if everything else has been eliminated through troubleshooting. We are talking about electronics here and there are basic rules to follow when dealing with those type of components.

Conclusion | Troubleshooting Carrier Thermostat Guide

This is a lot for a novice. If you are not comfortable doing it then call in an HVAC professional. Some of the things are basic and you could possibly do them yourself. However, some people don’t like dealing with anything electrical even low voltage. That is a detail of everything that could be causing the problem. It is one or the other. Troubleshooting issues like this is a process of elimination.

The Carrier thermostat is no different than many other thermostats. While some manufacturers do make their own thermostats, some simply get Honeywell to put their brand name on a Honeywell thermostat. A thermostat is simply an automatic switch that turns things on and off by temperature or manual input. Some thermostats are more complex than others but they all serve the same function. Therefore, troubleshooting a Carrier thermostat should be no different. Good luck Jeff!!

High Performance HVAC Heating & cooling

Carrier Thermostat Intermittent Problem


Resource: The Home Comfort Book: The ultimate guide to creating a comfortable, healthy, long lasting, and efficient home

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