Thermostat displays E1 and will not communicate with the indoor unit; no system response to mode changes.

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breakers before touching any wiring. Use a non-contact voltage tester to confirm power is off.

• Step 1 – Reboot the system: Turn off HVAC breakers for 2–3 minutes, then restore power. Wait 5 minutes to see if E1 clears.
• Step 2 – Inspect thermostat base wiring: Remove the Carrier smart thermostat from its wall plate. Verify each low-voltage wire is fully inserted and tightened under the correct terminal (e.g., R to R, C to C, Y1/Y to Y, W/W1 to W, G to G, or ABCD bus if using Carrier communicating system). Reseat any loose wires.
• Step 3 – Check for damaged cable: At the air handler/furnace control board, inspect the thermostat cable for cuts, crushed sections, or rodent damage. Gently tug each conductor at the board and thermostat to ensure it is not broken inside the insulation.
• Step 4 – Ohm test the conductors (advanced DIY): With power still off, disconnect thermostat wires at both ends. Use a multimeter in continuity mode to test each conductor from thermostat to furnace. Any conductor without continuity or with continuity to another conductor indicates a short or open—replace the thermostat cable.
• Step 5 – Inspect Carrier control board: Look for burnt spots, swollen components, or corrosion on the indoor control board. Reseat the thermostat harness/ABCD connector. If damage is visible or E1 persists with known-good wiring, the control board may need replacement.
• When to call a pro: If you are not comfortable testing low-voltage circuits or suspect a failed Carrier communicating control board, contact a Carrier-authorized technician.