How to Fix Honeywell Thermostat Red Light

If you are wondering what causes a honeywell thermostat red light indicator on one corner of your thermostat then this is a perfect guide for you.

Generally, a  red light on a honeywell thermostat will indicate problems with the AC condenser unit. The red light will also flash when the emergency heat on your honeywell is manually turned on to ensure that you are aware that the emergency heat is on.

When the L terminal is connected to the equipment monitor or a zone controller and something is not working right, , the sensor will send a signal  to the thermostat about a malfunctioning outdoor unit.

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However, you may  have noted that there is usually no red light when auxiliary heat automatically turns on,  on a heat pump. But the red light indicator will only come on when you manually run on emergency heat.

Below are different issues that can cause the red light to flash on your honeywell thermostat

Possible causes for a honeywell thermostat red light

Possible cause	Main fix
You have manually turned on Emergency Heat	No action is required
Condenser unit blocked	Unblock condenser unit
Malfunctioning compressor capacitor	Replace capacitor
Malfunctioning AC contactor	Replace ac contactor
Malfunctioning condenser fan	Replace fan motor
Low refrigerant	Sufficiently charge your ac

1) You have manually turned on Emergency Heat

The first thing you want to check when your honeywell thermostat is flashing a red light is look at the settings  on your thermostat and ensure that you have not turned on emergency heat.

 On most thermostats that are connected to zone controllers  or equipment monitors,  when emergency heat is manually turned on, it will flash a red light to remind you that emergency heat is on. 

This is mainly because emergency heat is  quite expensive and you don’t want to be surpassed by hefty electricity bills.

However, when the heat pump is running in the heating mode and auxiliary heat/emergency heat turns on automatically, you will most likely not see the red light. The red light only comes on when you manually turn on Emergency heat.

2) Blocked condenser unit

If you have not manually turned on emergency heat but there is a Red LED light blinking on your thermostat, check whether or not the outdoor ac unit is blocked by objects. Over time,  condenser units can become dirty and even get blocked with debris and all sorts of objects.

And when this happens your heat pump will not function as it ought. And if  your equipment is  connected to the monitors, it will definitely trigger the flashing led light on your thermostat to alert you if there is anything.

It is for this reason that you need to keep the ac condenser clean and free of debris. At Least  have an HVAC tune up  twice a year to ensure your system is always working in great conditioner.

In short when the condenser is blocked as seen in the image below clean it and  run your ac again. That should fix the red light.

3) Malfunctioning condenser fan

Another thing thing you want to check when honeywell is flashing a red light is check that your AC condenser unit is turning on and the fan is spinning. If the fan is not spinning you may have a defective fan motor.

A defective fan  motor is replaceable. But you will need to test it for continuity to ensure it is one causing the problem. It’s even better to call a professional if your condenser fan is not spinning.

On the other hand, you could have a red light on your honeywell and your fan  is working well. If that is the case, the problem could be with the compressor capacitor.

4) Defective AC capacitor 

That is another good reason you are seeing a red light on your thermostat. A defective capacitor will either prevent your compressor from starting or the fan motor from starting( if it’s a duo capacitor).

The best thing to do is open the condenser unit and physically inspect your capacitor. And many times it turns out, a defective capacitor will have a bulged top.

However, just looking at it can be deceiving. You need to test it for continuity. As a safety measure, always ensure that your capacitor is discharged as it can store charge that can pose danger.

If you have a dead capacitor, it can easily be replaced, and the good part is that capacitors can even be bought for cheap online.

5) Low refrigerant

When the ac condenser unit is running low on the refrigerant, the sensor will quickly pick up this and send a signal to the thermostat. The L terminal will then be energized and cause your thermostat to display a red light.

The refrigerant is an important component of your heat pump in that it absorbs heat and helps with transferring the heat between the outdoor and the indoor environments. If you are not a DIYer I wouldn’t recommend taking the job of charging your system with the refrigerant.

This is because when your system is running low on the refrigerant it will typically also have leaks. And you may also need to be licensed to handle the charging of your system.

So it will be best to call local professionals to address the issues if you believe your system could be running low on the refrigerant.

6) Malfunctioning AC contactor

When you are faced with a red light flashing on your thermostat,  a condenser unit may not be working and this could be caused by a  malfunctioning ac contactor or one that is stuck in the open position.

An AC contactor is a small device at the condenser unit that completes. It has two contacts, and  an electromagnet that pulls the contactor contacts together when a 24 volt from the thermostat energizes the electromagnet.

 When there is a contact, the 24VAC circuit is completed and  causes  the condenser to start working.

It is not unusual for the AC contactor to get stuck (especially if it has served for some years) , and when this happens, it prevents the condenser from turning on. 

This ac contactor failure could trigger the sensor to send a 24v signal to the L terminal of your honeywell thermostat and it will cause the red light to display.

Defective ac contactor should be replaced. They don’t cost much and they can easily be replaced.. AC contactor units are readily available and they can also be purchased online. Just make sure you check the model number to ensure that you purchase the right one.

Many homeowners find this hard to do and would rather hire a professional to fix for them. But if you can do it yourself it could save you a couple hundred dollars.

Read also: Honeywell hz311 troubleshooting Red Light [SOLVED]

Final Thoughts on Fixing a Honeywell Thermostat red Light

While we have seen possible causes for the honeywell thermostat red light,  all reasons and possible fixes may apply to other thermostat brands.

But If you are still facing the problem after trying out all the tips, it’s perhaps a good idea to call local professionals for help