Thermostat shows E20 and indicates C‑wire or power stealing configuration error; device may reboot when HVAC starts.

Safety: Turn off power to HVAC equipment before modifying wiring.

• Step 1 – Check for C wire: Remove the thermostat and look for a wire connected to the C terminal. If none is present, check behind the wall and at the furnace control board for an unused conductor that can be connected to C.
• Step 2 – Connect C properly: At the furnace/air handler control board, land the chosen conductor on the C terminal. At the thermostat, connect the same conductor to C.
• Step 3 – Verify connections: Ensure all low‑voltage connections are tight and that no bare wires are touching.
• Step 4 – Adjust power stealing settings: If your Johnson Controls thermostat has a power stealing or load configuration option, set it according to the HVAC manufacturer’s recommendations or disable it when a solid C wire is present.
• Step 5 – Test operation: Restore power and run heating and cooling cycles. If E20 persists or the thermostat still reboots, consult a technician to evaluate transformer capacity and wiring.