How To Fix A Furnace Error Code 12

In any home with a forced-air heating system, the furnace is the central hub for all things heat-related. The furnace will have various sensors, switches, and indicators to monitor heat levels and attic temperature. If you see an “Error Code 12” message on your furnace display panel, then you likely have an issue with your return air sensor. This article will explain what Error Code 12 means on a furnace and how to troubleshoot this problem as well as identify its cause.

What does Error Code 12 mean?

Error Code 12 means that there is a problem with your furnace’s return air sensor circuit. In other words, this error code is telling you that the furnace thinks there is a problem with the wiring or the sensor. The return air sensor is a device responsible for measuring the temperature of return air. This measurement is then used as a basis to control the furnace’s burn rate. In simple terms, the return air sensor is like an indoor furnace thermostat. More specifically, it’s a temperature switch that activates the furnace when the indoor air is too cold.

How to troubleshoot Error Code 12?

If the display panel on your furnace is showing error 12, you’ll need to troubleshoot the problem to ensure your furnace is operating correctly. First, check and make sure that the return air sensor is properly installed in the ductwork. If the sensor is not installed correctly, you may need to re-install it. Next, check the circuit wiring for the return air sensor for any signs of damage, wear, or corrosion. Also, make sure to check for any loose connections as well. If there is an issue with the wiring, you may want to consider replacing your return air sensor. If the sensor appears to be in good condition and connected correctly, you’ll want to check the furnace’s limit/reset button. This button will reset the furnace and clear any error codes.

Reset the furnace

If you’ve checked the wiring and sensor, and there appear to be no issues, you can clear the error code by pressing the reset/limit button on the furnace. You can find this button on the furnace’s control panel. This button may be labeled as “Reset”, “Limit”, or “Clear”. Simply press this button and wait for the error code to disappear from the display panel. If the error code doesn’t go away after pressing the reset button, you may need to replace the sensor.

Change your Furnace Air Filter

If you’ve checked your wiring and sensor, and you still have an error code, you’ll want to double-check that your air filter is clean. Dirty air filters can cause your furnace to overheat, which can lead to an overheating sensor that will trigger the error code. You should clean or replace your air filter at least once a month. If your filter is more than one month old, it should be replaced. If your filter is very dirty (e.g. covered in dust), you should change it immediately. If changing your filter doesn’t resolve the error code, you may need to replace the sensor.

Check Your Ductwork for Blockages and Leaks

A clogged or blocked ductwork can trigger an error 12 on your furnace. This is because a blockage in the ductwork will cause the return air sensor to overheat. Once the sensor is overheated, the furnace will trigger the error code. To check for a ductwork blockage, you’ll need to remove the furnace’s ductwork section and inspect it for any signs of clogs or blockages. You may also want to check for any signs of water damage, broken joints, or loose connections. If you find a blockage or a leak in the ductwork, you’ll need to clear it as soon as possible. Failure to do so will just make the problem worse.

Re-install your return air sensor(s)

If the sensor is installed correctly, the wiring is in good condition and the sensor is clean, you may need to replace the sensor. You may want to check the furnace’s model number and/or manufacturer to find out which sensor to buy. Before you re-install the sensor, you need to reset the furnace. Once the sensor is re-installed, you should be able to clear the error code. If the error code doesn’t go away, you may need to replace the entire section of ductwork.


Furnace error codes are typically indicative of a problem with the furnace. In most cases, the error code will point you in the right direction to the faulty component. However, sometimes you may need to replace a few things before the furnace starts working again. If you’re experiencing issues with your heating system, it’s best to call a professional as soon as possible. This will allow the technician to figure out the issue and fix it as quickly as possible.