American Standard
Loss of 24V Power to Thermostat
Thermostat display is completely blank and system does not run
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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 furnace control board, Tripped HVAC breaker, Broken R or C wire, Failed 24V transformer in American Standard air handler or furnace
How to Fix / Troubleshooting
Safety: Turn off the HVAC breaker before removing furnace or air handler panels. Use caution around 120/240V wiring.
- Check breakers: Verify that the furnace/air handler and outdoor unit breakers are ON. Reset any tripped breakers once.
- Inspect furnace fuse: Remove the blower door and locate the low-voltage fuse (typically 3–5A automotive-style) on the American Standard control board. Replace if blown, but if it blows again, there is a short that must be found.
- Check thermostat wiring: At the furnace board, ensure R and C wires going to the thermostat are secure. Look for shorts where the cable passes through sheet metal.
- Measure voltage: With power on and using a multimeter, measure between R and C at the furnace board. If no 24V is present, the transformer may have failed and should be replaced by a technician.
Note: Do not bypass fuses or use larger fuses; this can damage the control board.
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Repair Difficulty
Hard
4/5
Required Part
American Standard furnace/air handler control board fuse or 24V transformer
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