Thermostat displays E02 and does not detect the C (common) wire; screen may dim or reboot when calling for heat or cool.

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

• Step 1 – Confirm wiring at thermostat: Remove the Geo thermostat from the wall plate. Verify the C wire is firmly inserted into the C terminal and the terminal screw is tight. Tug gently to ensure it is secure.
• Step 2 – Inspect wiring at HVAC control board: At the furnace/air handler, remove the access panel. Locate the low‑voltage control board. Confirm the same C wire is securely attached to the C terminal and not broken or corroded.
• Step 3 – Check low‑voltage fuse: Look for a 3–5 A automotive‑style blade fuse on the control board. If blown, replace with the same rating. If it blows again, there is a short in the thermostat or wiring that must be found before further replacement.
• Step 4 – Measure voltage: Using a multimeter set to AC volts, measure between R and C at the control board and at the thermostat wall plate. You should see ~24 VAC. If present at the board but not at the thermostat, the cable is open or damaged.
• Step 5 – Bypass damaged conductor: If one conductor is open, repurpose an unused spare conductor in the thermostat cable as the new C wire, connecting it at both ends.
• Step 6 – Consider a Geo power extender: If you do not have a C wire and cannot run one, install a Geo‑compatible power extender kit at the furnace control board per the kit instructions.
• When to stop: If you are not comfortable working inside the furnace cabinet or measuring live voltage, contact an HVAC technician to restore proper 24 VAC power.