Thermostat displays E1 and will not control heating or cooling

Safety first: Turn off power to the HVAC system at the breaker before removing the thermostat from the wall plate.

Steps to troubleshoot:

• 1. Power cycle: Turn the HVAC breaker off for 60 seconds, then back on. If E1 clears temporarily but returns, continue below.
• 2. Remove thermostat from wall plate: Gently pull the Resideo thermostat straight off the wall plate to expose the wiring and terminals.
• 3. Check for remote sensor: If your model supports a remote indoor sensor (e.g., Resideo C7189U1005), locate the sensor wires on the S1/S2 or similar terminals. Ensure both wires are firmly clamped and not broken.
• 4. Inspect wiring: Look for cuts, staples through the cable, or corrosion on the sensor wires. Tighten any loose terminal screws.
• 5. Test sensor resistance: With power still off, disconnect the sensor wires and measure resistance across the sensor with a multimeter. Compare to the Resideo temperature/resistance chart in the installation manual. An open (infinite) reading indicates a failed sensor.
• 6. Bypass test: Temporarily connect a known-good Resideo indoor sensor or a resistor of the correct value across the sensor terminals. If E1 clears, the original sensor or its wiring is faulty.
• 7. Replace failed component: Replace the remote indoor sensor or the thermostat (if the internal sensor is open) following the Resideo installation guide.

When to call a professional: If you are not comfortable using a multimeter or the E1 code persists after sensor replacement, contact a licensed HVAC technician or Resideo support.