Furnace shows E20; unit may not respond to thermostat or behaves erratically

Safety first: Turn off power before adjusting thermostat wiring.

• Inspect connections: Remove the furnace blower door and check the thermostat wires at the control board (R, W, Y, G, C). Tighten any loose screws and ensure no bare wires are touching.
• Bypass thermostat (test only): With power on, briefly jumper R to W at the control board. If the furnace starts normally, the thermostat or its wiring is suspect.
• Check thermostat settings: Ensure the thermostat is set to HEAT mode and configured for a gas furnace (not heat pump) per the thermostat manual.
• Replace thermostat: If wiring is intact and the furnace operates correctly when jumpered, replace the thermostat with a compatible model.
• When to suspect board: If the furnace does not respond even when R and W are jumpered at the board, the control board input circuit may be defective.