Thermostat displays E18 and indicates a dehumidifier or humidifier accessory fault.

Safety first: Turn off power to the HVAC system and any accessory devices at the breaker before working on wiring.

• Step 1 – Identify accessory: Determine whether a humidifier, dehumidifier, or ventilation device is connected to the Geo thermostat's AUX or HUM terminals.
• Step 2 – Verify wiring: Compare the accessory wiring to the Geo installation manual. Ensure the accessory is connected to the correct low‑voltage terminals and not directly to high‑voltage outputs.
• Step 3 – Check accessory coil resistance: Disconnect the accessory leads and measure resistance across the coil with a multimeter. A very low resistance may indicate a shorted coil.
• Step 4 – Test without accessory: Leave the accessory disconnected and power the system back on. If E18 clears, the accessory or its wiring is the cause.
• Step 5 – Use an isolation relay: If the accessory current draw exceeds the thermostat's AUX rating, install an isolation relay powered by the HVAC transformer, controlled by the Geo AUX output.
• Step 6 – Replace thermostat if AUX driver failed: If wiring and accessory are correct but E18 persists, the AUX driver on the Geo PCB may be damaged. Replace the thermostat head unit.