Thermostat shows E12 and may indicate 'Aux heat lockout' or 'Auxiliary heat control error' on heat pump systems.

Safety first: Turn off power to the air handler/furnace at the breaker before working on wiring.

Steps:

• Verify system configuration: In the Rheem thermostat installer setup, confirm the system type is correctly set (heat pump with aux heat, conventional, etc.) and that the number of heat stages matches the equipment.
• Check W2/Aux wiring: Ensure the W2/Aux terminal on the thermostat is connected to the correct W/Aux terminal on the indoor unit control board.
• Test auxiliary heat: With power restored, raise the setpoint significantly to force auxiliary heat. Check if the indoor unit energizes the heat strips or furnace. If not, measure 24VAC between W2/Aux and C at the board when aux heat is called.
• Inspect fuses and breakers: For electric heat strips, verify that the dedicated breakers are on and any internal fuses are intact.
• If thermostat output is present but aux heat does not run: The issue is in the indoor unit (heat relay, sequencer, or heat strips). If no output from thermostat, the thermostat's aux heat driver may be faulty and require replacement.