Thermostat displays E16 and indicates a short circuit on one of the control outputs; HVAC system may be locked out.

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breakers before inspecting wiring.

• Step 1 – Inspect thermostat cable: Remove the thermostat and carefully inspect the cable where it exits the wall and enters the wall plate. Look for nicks, crushed insulation, or exposed copper.
• Step 2 – Check furnace entry point: At the furnace/air handler, inspect where the thermostat cable enters the cabinet. Ensure the metal edge has a grommet and has not cut into the cable.
• Step 3 – Isolate circuits: Disconnect all thermostat wires from the Geo wall plate except R and C. Reattach the thermostat and see if E16 clears. Then reconnect one wire at a time (W, Y, G, O/B, AUX) until the error returns; the last wire connected is on the shorted circuit.
• Step 4 – Repair or replace cable: For the identified circuit, inspect the full cable run if accessible. Repair damaged sections or run a new cable if necessary.
• Step 5 – Check accessories: If the short is on AUX or HUM terminals, verify any connected humidifier or accessory is wired per Geo's diagram and not drawing excessive current.
• Step 6 – Replace thermostat if output is damaged: If wiring is corrected but E16 persists on a specific output, the corresponding triac/relay on the Geo PCB may be damaged. Replace the thermostat head unit.