Thermostat shows E18 and indicates internal battery or backup power fault (on models with backup battery)

Safety first: Remove power from the thermostat by turning off the furnace/air handler breaker before opening any battery compartment.

Steps to fix E18 backup power fault:

• 1. Access battery compartment: Remove the thermostat from the wall and open the battery compartment if present.
• 2. Inspect and replace battery: Check for corrosion or leakage. Replace the battery with the exact type specified by Goodman (e.g., CR2032 coin cell) and observe polarity markings.
• 3. Clean contacts: If corrosion is present, gently clean contacts with a cotton swab and isopropyl alcohol. Allow to dry completely.
• 4. Reinstall and power up: Reassemble the thermostat, restore power, and verify that E18 clears.
• 5. Replace thermostat if needed: If a new battery does not resolve the error, the charging or sensing circuit may be defective and the thermostat should be replaced.