E8
American Standard
Output Relay / Stage Control Fault
Thermostat shows E8 and cooling or heating stages do not respond correctly
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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
Stuck or failed relay on thermostat control board, Shorted Y1/Y2/W1/W2 output, Miswired staging terminals, Overcurrent from contactor coil or gas valve
How to Fix / Troubleshooting
Safety: Turn off power to indoor and outdoor units at the breakers before working on control wiring.
- Visual wiring check: At the thermostat, verify that Y1, Y2, W1, W2, G, and O/B (if used) are wired according to the system type (heat pump vs. conventional). Correct any miswiring.
- Isolate outputs: Disconnect one suspect output wire (e.g., Y1) at the thermostat and restore power. If E8 clears, the issue may be in the downstream device (contactor coil, wiring) rather than the thermostat.
- Check for shorts to ground: Inspect the low-voltage wiring at the outdoor unit contactor and furnace control board. Look for insulation damage or wires touching metal.
- Test operation: With wiring corrected, command heating and cooling from the thermostat and listen for relays clicking. If the thermostat does not energize outputs but still shows E8, the internal relay board may be defective.
Resolution: A failed output relay typically requires thermostat replacement.
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Repair Difficulty
Hard
4/5
Required Part
American Standard smart thermostat relay/output board
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