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Problem: Fan will not come on in the “On” position or your fan will not come on when you turn the thermostat down


Problem: Fan will not come on in the “ON” position or your fan will not come on when you turn the thermostat down in the air conditioning mode.

Solution: Test to make sure you are getting 24 to 28 volts AC between the G (green) and C (com) terminals on you fan center or control board. If you aren’t getting 24 volts between G and C then you might have a thermostat problem, a thermostat wire problem or the low voltage fuse on the control board might be blown. Below we have a picture of a control board that shows the low voltage fuse. If you think you need a control board please email us with your furnace’s model number and we will be glad to try a find the right board for your furnace. Our email address is: You might need a new fan center relay (if your furnace is equipped), fan sequencer or furnace control board.


A sequencer looks like the picture below. Sequencers are used most of the time in air handlers (heat pumps, electric furnaces) to control the fan blower and turn on the electric heaters in a sequence so the electric heaters to do come on all at the same time. Most of the newer air handlers have electronic circuit boards. You should have continuity between M1 and M2 when 24 volts Ac is applied to the bottom terminals. We have sequencers for sale on our site. Please search for “sequencers”.

2 Stack Sequencer

Purpose of the Fan Center Relay: The purpose of the fan center relay is to provide a low voltage control (24 Volts) to control a relay that can be used to control two-speed blower motors and auxiliary circuits in heating or cooling equipment. Many of the newer furnaces do not have fan center relays, but many of the older furnaces and some new furnaces still do. You would need to look inside your furnace to determine if you have a fan center relay. Some of the relays are attached to a junction box where the power comes into the furnace, and others are located in the blower compartment attached to the blower housing. You might not have the relay that we sell below. Many of the newer furnaces are control by electronic circuit boards. We sell the White-Rodgers fan control center on our site. Please search for “fan center”. If you think the control board is causing the fan to not come on please send us your furnace’s model number and we will try and find out which control board your furnace uses. Our email address is:

Troubleshooting procedure:

1. Make sure that your furnace switch is “ON” the furnace blower compartment door is on and secure. Move the lever on the thermostat to the “ON” position. The fan blower should come on. If it doesn’t then you probably have loose or burnt wires in behind the relay or you may need a new fan relay or circuit board.
2. Many times wires are either loose or burnt behind the fan center relay. First make sure your furnace power is turned off. Remove the fan center relay and inspect it for loose or burnt wires. Repair wires and reinstall fan center relay. Turn the power to the furnace back “ON.” Hopefully you have fixed the problem!
3. With the furnace on use a multimeter set to AC Volts, to test between terminals “R” and “C.” You should get a voltage reading of approximately 24 volts AC. If you do not get any voltage reading, your low voltage transformer might need replacing or you are not getting high voltage to power the transformer. Sometimes furnace manufacturer’s connect the furnaces transformer through the limit control so if your furnace over heats or if the limit is stuck open then you will not get power to the transformer. There are quiet a few variables that can occur to cause furnace problems. I believe I have covered the main problems. If you have the fan center relay like we sell below the low voltage transformer is combined with the fan relay. This relay will work on motors up to 3/4 horse power.

2 thoughts on “Problem: Fan will not come on in the “On” position or your fan will not come on when you turn the thermostat down

  1. I have a gas heater with an electric fan. I just had my roof replaced a week ago this Saturday and it worked fine up until this point .

    Would replacing cause my fan not to come on? I know it’s probable a silly question but again it was working fine up until this point

    I appreciate your reply

    Sincerely ,

    1. Hi Annie! Replacing the roof could only cause the fan to not come on if the roofers accidentally cut or stepped on your thermostat wires and caused a break in the wires. They could have loosened a connection up on your control board or thermostat with all the banging and vibration from hammering the roofing nails. I would suggest that you turn the power off to the furnace and make sure all wire connections are good and tight. Turn the power to the furnace back on, turn the thermostat to the “Fan ON” position and see if you are getting 24 volts between the G (Green) wire and the C (Common) wire. If you are not getting 24 volts then you either have a thermostat problem, thermostat wire problem or you might have a blown fuse on your control board. If you are getting 24 volts between C and G and the fan is not running then you might have a control board problem, fan relay problem or a bad fan motor or bad fan motor capacitor. We have a really good Youtube video on the following page that tells about the top 10 reasons why the blower fan motor will not come on. Here is the link: I hope you can easily find and fix the problem. Steve

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