So your indoor fan is working non-stop even though you have your AC or your furnace OFF. I know it can be really frustrating if you encounter this situation. 

As an ac technician, I get so many calls around the years about fan problems. And I know what are the most common reasons why your fan is continuously running.

So In this article, I’ll list all the problems why your ac fan keeps running and their DIY solutions. 

8 Reasons Why Your AC Fan Keeps Running

Reason No#1: Thermostat Fan Setting On

The first reason why your ac fan is always running is because of the thermostat settings. Your thermostat has three fan settings AUTO, ON, AND OFF. Most of the time fan setting is at AUTO.

But a lot of people don’t realize that thermostats settings are sometimes bumped to the ON. And the difference between auto and on is when it's AUTO the fan inside of the furnace will only turn on with the furnace.

Whenever the furnace and air conditioner turns on the fan turns On and when it turns off the fan turns off. But if you set it to ON then the fan will run 24/7. So make sure to check your thermostat settings.

Reason No#2: Blown 3 or 5 Amp Fuse

Reason number 2 is a blown 3 or 5 Amps fuse. This fuse is mostly located somewhere around the control board. Sometimes this fuse gets blown which can cause the fan to run all the time.

You can check this fuse by opening your furnace. If you see a burn mark on the fuse then it’s probably blown and needs to be replaced.

Reason No#3: Stuck High Limit Switch

Reason number three is a stuck high limit switch. So the high limit usually looks like a little oval-shaped black plastic disc with metal prongs coming out of it. But most of the time it'll be a black rectangle-shaped box.

The high limit is basically a switch that trips if your furnace is overheating. If your furnace is running and for some reason, it's overheating that high limit switch trips or it opens up and that shuts the burners off but the fan continues to run.

And what happens is sometimes this switch will get stuck in the open position. So it'll open up and it will not close back up oftentimes.

You can just take a screwdriver and just give it a good whack and that’ll open it up. But if that switch continues to stuck then you’ll have to replace it.

Reason No#4: Tripped Flame Rollout

Reason number four is another limit except this one is called a flame rollout. When this rollout switch trips it can cause your furnace fan to run continuously.

A lot of times these flame rollouts will be around the burners somewhere. If you have a sealed burner box it'll typically be either in the front bottom or on the sides. 

A flame rollout switch is a manual reset switch which means if this thing ever trips it'll not automatically reset, unlike the high limit switch.

In normal operating conditions the high limit switch will automatically reset once the furnace cools off and everything will continue to work normally. But on the rollout switch if this ever trips you actually have to manually reset it. 

Check Our Guide On Troubleshooting Furnace Not Igniting.

Reason No#5: Tripped low-pressure Gas Switch

Reason number five is a tripped low-pressure gas switch. This typically only applies to LP furnaces (propane furnaces) most of them will have a Gas switch.

So the low-pressure gas switch will typically be right before the gas valve or it'll be mounted on the side of the gas valve. So sometimes a furnace will be set up in a way where if that switch trips the blower fan will come on and just stay on continuously.

And it could be an indication that your LP Tank is low on gas and that's why that thing is tripping. You’ll have to call a technician for this problem because it’s not really a DIY type of thing.

Reason No#6: Bad Thermostat

Reason number six is a bad thermostat. A bad thermostat can cause your blower motor to run all the time. Now there's a lot of hard ways to check a thermostat you can use a meter you can do Hot wiring.

But really the easiest way to check it is to simply take that thermostat and pull it off the base. If the blower motor turns off when you pull the thermostat off that means that your thermostat is bad.

But if you take that thermostat off the wall and the blower motor continues to run then you know that the thermostat is not the problem.

Check Our Full Tutorial On How To Check and Bypass The Thermostat

Reason No#7: Shorted wires

The seventh reason why your blower motor can continually be running and not turning off is that you have shorted wires.

And typically the shorted wires that I'm talking about are the G wire is touching the R wire somewhere inside of your walls or maybe at the thermostat itself or at the control board in your furnace. Usually, the green wire will typically go to G(Fan) and then the red wire will typically go to R which is power.

So G is fan R is power so if somewhere the G wire gets chewed and they're touching each other that just means that power will be continuously going to G which is a fan and that will turn on the fan and keep it on all the time.

The easiest way to check is to pull your R wire the red wire and the G wire (green) of the control board. And then using your meter just check continuity between R and G and it should be OL.

If you have resistance or continuity between those two wires then you know for sure you have those two wires shorting out between each other somewhere. If it’s too hassle for you then just call a technician and he can do that for you.

Reason No#8: Bad Control Board Fan Relay

Reason number eight is a stuck fan relay on a control board. So essentially the whole board is bad or if you can somehow track down the relay and replace just the relay that could solve your problem.

So long story short if you ruled everything out and if you don't have power between G and R. So if you would take your meter leaves and with the power on if you would check from G to common.

If you don't have any voltage there that means the fan should not be running. But if it is running that means that the relay is stuck closed. So it's just a little switch that opens and closes and with time the switch can get stuck.

You can try smacking it if it works then good for you. Like I mentioned previously if it keeps seizing up like that and locking up. Then you just have to replace the board or if you're lucky and you can find a new fan relay you could probably solder a new one in.

Also Check Our AC Repair Guide: Most Reported Problems by homeowners and their DIY Solution