E11 displayed and thermostat cannot control heat pump reversing valve (no change between heat and cool)

Danger: Turn off power to both indoor and outdoor units before working on wiring.

• Step 1 – Verify configuration:
  • In thermostat settings, confirm system type is set to Heat Pump.
  • Set O/B operation correctly (energize in heat vs cool) per your heat pump manufacturer.
• Step 2 – Inspect O/B wiring:
  • Remove the thermostat and check that the O/B wire is firmly connected and not shorted to other terminals.
  • At the outdoor unit, verify the O/B terminal connection is secure.
• Step 3 – Test O/B output:
  • With power on and a call for heat or cool active, measure voltage between O/B and C at the thermostat.
  • You should see ~24VAC when the reversing valve is commanded on.
  • If no voltage is present but R/C are good, the O/B driver on the thermostat may be faulty.
• Step 4 – Check for shorts:
  • If the low‑voltage fuse at the air handler keeps blowing when O/B is connected, inspect the cable for shorts to ground or other conductors.
• Step 5 – Replace thermostat if O/B driver is defective:
  • Internal O/B output components are not serviceable; replace the thermostat if E11 persists with known‑good wiring.