Thermostat displays E24 and indicates heat pump reversing valve fault

Safety first: Turn off power to indoor and outdoor units at the breakers before adjusting wiring.

Steps to troubleshoot:

• 1. Confirm system is a heat pump: Check the outdoor unit model or existing thermostat labels to verify a heat pump system.
• 2. Verify O/B wiring: At the thermostat, ensure the O/B wire is connected only to the O/B terminal. At the furnace/air handler or outdoor unit, confirm the same conductor is connected to the reversing valve control terminal.
• 3. Check O/B configuration: In the installer setup, set O/B to match your heat pump (O for cool-energized, B for heat-energized). Refer to the outdoor unit documentation.
• 4. Inspect for shorts: Look for damaged thermostat cable where it passes through walls or near metal edges.
• 5. Test operation: After correcting wiring and configuration, test both heating and cooling modes to ensure the system switches correctly.
• 6. Replace thermostat if O/B relay is defective: If the reversing valve never energizes despite correct wiring and configuration, the thermostat relay may be faulty.

When to call a professional: If the heat pump still does not switch modes correctly, an HVAC technician should test the reversing valve coil and outdoor unit control board.