07
American Standard
Low Voltage Lockout / No 24V from Transformer
Error code 07 and furnace will not respond to thermostat, no inducer or blower operation
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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
Blown low-voltage fuse on integrated furnace control, Shorted thermostat wiring, Failed 24V control transformer, Miswired accessory (humidifier or air cleaner) on R and C terminals
How to Fix / Troubleshooting
Safety: Turn off power before accessing the control board or transformer.
Error 07 often indicates a low-voltage problem or blown fuse.
- Check the 3–5A blade fuse on the control board:
- Remove the blower door to access the integrated furnace control.
- Inspect the automotive-style fuse; if blown, replace with the same amperage.
- If it blows again immediately, there is a short in the 24V circuit.
- Inspect thermostat wiring:
- Look for bare wires touching the furnace cabinet or each other, especially near the board and at the thermostat.
- Repair damaged insulation and secure wires.
- Test transformer output:
- With power on and extreme caution, measure between R and C terminals; you should see ~24VAC.
- If there is no voltage and the primary side has power, replace the 24V transformer with an American Standard-compatible unit.
- If you are not comfortable working with live voltage, contact a professional technician.
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Repair Difficulty
Hard
4/5
Required Part
Integrated Furnace Control Board
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