Thermostat display shows E0 and will not communicate with Goodman furnace or air handler

Safety first: Turn off power to the HVAC system at the furnace/air handler switch and the breaker panel before touching any wiring.

Steps to troubleshoot E0 communication fault:

• 1. Power down the system: Switch off the furnace/air handler disconnect and the thermostat (if it has a power switch). Wait 1–2 minutes.
• 2. Inspect thermostat wiring: Remove the thermostat from its wall sub-base. Verify the A, B, and C (or R/C) terminals are correctly landed and firmly tightened. Look for broken, pinched, or corroded wires.
• 3. Check polarity: At both the thermostat and the Goodman furnace control board, confirm that the A wire is on A and the B wire is on B. Reversed A/B will cause communication errors. Correct any miswiring.
• 4. Test continuity: With power still off, use a multimeter to check continuity of the A and B conductors from the thermostat to the furnace control board. Repair or replace any open or shorted cable runs.
• 5. Inspect furnace control board terminals: Open the furnace blower door, locate the Goodman communicating control board, and verify the A/B/C terminals are tight and not corroded or burnt. Reseat the low-voltage harness if present.
• 6. Power cycle and reset: Restore power to the furnace/air handler and then to the thermostat. Perform a system reset from the thermostat menu if available. Allow a few minutes for the devices to re-establish communication.
• 7. Replace suspect components: If wiring is confirmed good and E0 persists, the communication circuit on the thermostat or the Goodman control board may be defective. Try a known-good thermostat first. If that fails, contact a professional to evaluate and possibly replace the furnace control board.