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Furnace Not Working After Changing Thermostat? How to Fix

If your furnace isn’t working after changing the thermostat, possible causes include a lack of power, a faulty thermostat battery, and issues with the furnace control board. If you replaced your thermostat to fix another issue with your furnace, you may have misdiagnosed the cause of the problem.

Rule Out the Thermostat As the Problem

Since the last change you made to your home’s heating system was changing the thermostat, you should start your diagnosis there.

  • Confirm you installed the correct thermostat. Thermostats are model-specific; installing the wrong one can cause a communication issue between it and the furnace. Do your research and consult a professional before purchasing a thermostat.
  • Be conscious of your thermostat’s location. You can’t expose the thermostat to sun rays or heat sources; they work by detecting heat changes, and if heat rays are constantly in contact with it, it will detect the wrong temperature. Any significant holes behind the thermostat also affect readings, so be sure to look for that as well.
  • Confirm the thermostat can get power. A thermostat has a display that lights up and shows the set temperature; if the display is off, there is no power connection. Find out whether electric power in the house or the power supply from the circuit breaker and main switch is turned off and switch it on.
  • Check the thermostat’s battery. Some thermostats use batteries, and although you just replaced them, you may have bought one with a faulty battery. Replace the battery and test it again.
  • Check the thermostat’s fuse. If the fuse is blown, the thermostat won’t work, and the furnace won’t get the “turn on” command. You can easily replace fuses once you know the make and model. To be safe, carry the damaged fuse to the hardware store to get the correct replacement.
  • Check if your room’s temperature is higher than your thermostat setting. When setting the thermostat to heat your room, ensure that you set a temperature slightly above the room temperature; otherwise, the thermostat won’t trigger the furnace to turn on. Reset your thermostat and wait a few minutes for the heat to kick in.

If the power supply works but the furnace still won’t work, you should investigate inside the thermostat. Here’s how you can do that:

  • You will have to turn the power supply off at the circuit breaker
  • Uncover the thermostat. You don’t need any tools for this, as the cover pops right up.
  • Check whether you left loose screws or wires in the thermostat while installing it and tighten them.
  • Close the cover back on, then switch on the power supply and test the thermostat.

Common Problems With Furnace After Changing The Thermostat

There are some common issues that I came across in my journey with my furnace after changing the thermostat. I will take you through some of those to help you identify them:

Wiring Mismatch

Incompatibility between the existing furnace wiring and the new thermostat can lead to communication breakdown. Different thermostat models may require specific wiring configurations. Mismatch wires or improper connection can hinder the thermostat’s ability to convey commands to the furnace.

Troubleshooting Steps

  • Double-check the thermostat installation guide and furnace manual.
  • Confirm each wire is correctly connected.
  • If there’s a mismatch rewire or use an adaptor if compatible.
  • Look for a professional if you are unable to fix the issue.

Wiring Mismatch

Some older furnaces may not be compatible with certain modern thermostats, this may cause your furnace to not respond to the thermostat. Newer thermostats can often have advanced features and communication protocols that older furnaces might not support. Checking compatibility before the switches is crucial to avoid disruptions.

Troubleshooting Steps

  • Verify the compatibility before investing in a thermostat.
  • If you find an issue, consider installing a relay.
  • If you are unsure, seek a professional to bridge the gap between the old furnace and the new thermostat technologies.

Power Supply Problems

Insufficient or disrupted power supply to the thermostat can impede its ability to communicate with the furnace effectively. Faulty wiring, blown fuses, or issues with the electrical system can interrupt the power flow. Confirming a stable power source is essential for the thermostat furnace connection.

Troubleshooting Steps

  • Inspect the wiring for any damage.
  • Check the fuse and circuit breakers.
  • Confirm a stable power supply by fixing any issues in the electrical system.
  • If problems persist, consult an electrician to address wiring or power supply problems.

Configuration Settings

ncorrect settings or programming on the new thermostat can lead to erratic furnace behavior. Overlooking the proper configuration of temperature thresholds, schedules, or fan settings can confuse the furnace’s response. Double-checking the thermostat settings is important for smooth operation.

Troubleshooting Steps

  • Review the thermostat settings.
  • Reconfigure the setting (if needed) according to the temperature.
  • Pay attention to the temperature thresholds, schedules, and fan settings.
  • Reset the thermostat to factory default if needed.
  • Consult the user manual for accurate configuration.

Sensor Calibration

Inaccurate temperature readings from the thermostat sensor can affect the efficiency of the thermostat. Misplacement or poor calibration of the thermostat sensor can lead to false temperature readings. The furnace may underheat or overheat based on the inaccurate information.

Troubleshooting Steps

  • Verify the sensor’s placement and calibration.
  • Confirm it is not exposed to direct sunlight or drafts that may affect temperature readings.
  • Recalibrate the sensor according to the thermostat’s guidelines to provide accurate data to the furnace.

Check For A Problem With Your Furnace Control Board

The control board ensures that individual components in the furnace have a smooth working sequence. A typical furnace can have any of two control boards, including ignition control boards and furnace controls. 

If your furnace is not turning on or malfunctioning, there might be a problem with the control board. Here is a table explaining the steps to check the problem with a furnace control board:

Visual InspectionsExamine the control board for any visible signs of damage, such as burnt areas, loose wires, or corrosion.
Check For PowerConfirm that the furnace is receiving power. Check circuit breakers and fuses to confirm that there are no issues with the electrical supply.
LED Indicator Lights A lot of furnace control boards have LED indicator lights. Check if there are any error codes or patterns displayed on the control board. Refer to the furnace manual for the interpretation.
Test VoltageUse a multimeter to measure the voltage across the control board. Lack of voltage may indicate an issue with the power supply or a faulty control board.
Inspect CapacitorsExamine capacitors on the control board. Lack of voltage may indicate an issue with the power supply or a faulty control board.
Test Relay SwitchesIf applicable, test relay switches on the control board. Clicking sounds indicate proper functioning. If silent, the relay may be faulty.
Inspect WiringCheck all wiring connections on the control board. Loose or disconnected wires can disrupt communication between the thermostat and the furnace.
Reset Control BoardReset the furnace by turning off the power for a few minutes and then turning it back on to reset the control board.

Test Whether Your Thermostat Works

You can test the efficiency to confirm the furnace and thermostat’s optimal performance, by following these simple steps.

Here are some step-by-step instructions:

  • Step 1: Set the thermostat to a temperature higher than the current room temperature.
  • Step 2: Listen for a click indicating that the thermostat has sent a signal to the furnace.
  • Step 3: Check if the furnace kicks in and starts heating up to the current readings.
  • Step 4: If the furnace doesn’t respond, verify the thermostat settings, wiring, and power supply.

If you encounter specific issues during this process, feel free to reach out to an expert for assistance.

Did You Misdiagnose The Problem?

The fact that you replaced the thermostat means that the furnace wasn’t working previously, and you may have mistakenly identified the thermostat as the problem. If none of the above solutions fixed your furnace, the thermostat probably wasn’t the issue. If you’re having this much trouble diagnosing your thermostat, I highly recommend hiring a furnace repair expert to take care of this.

Professional Help

Seek professional help when troubleshooting your furnace becomes complex or when issues persist despite basic interventions. If you encounter challenges identifying wiring problems, configuring the thermostat, or addressing compatibility issues, it’s advisable to consult an HVAC technician. 

Trying to perform DIY repairs yourself any further can cause more damage and expenses. Additionally, if a visual inspection reveals any damage to the control board, capacitors, or wiring that exceeds your expertise, professional assistance is important.

HVAC professionals possess the knowledge and tools to conduct thorough assessments, pinpoint underlying issues, and implement effective solutions. They confirm the safe, time-saving, and efficient operation of your heating system to restore the comfort in your home.


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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