E18
American Standard
Indoor Fan (G Terminal) Control Fault
Thermostat shows E18 and fan control is erratic or stuck on/off
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Shorted G wire to R, Failed fan relay in thermostat, Miswired G terminal at furnace board, Stuck fan relay on furnace control board
How to Fix / Troubleshooting
Safety: Turn off power to the furnace/air handler at the breaker before working on fan wiring.
- Check thermostat G wiring: Remove the thermostat and inspect the G terminal. Ensure the wire is secure and not touching R or other terminals.
- Inspect furnace control board: At the air handler/furnace, verify that the G wire from the thermostat is connected to the G terminal and that there are no additional jumpers causing constant fan operation.
- Test fan mode: With power restored, switch the thermostat fan mode between Auto and On. If the fan does not respond but other functions work, the thermostat’s fan relay may be defective.
- Bypass test (for pros): A technician can jumper R to G at the furnace board to verify the blower operates correctly, isolating the thermostat as the fault.
Note: If the furnace board relay is stuck, the board itself may need replacement.
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Repair Difficulty
Hard
4/5
Required Part
American Standard thermostat fan relay or furnace control board G relay
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