Thermostat displays E1 and system will not run in heating or cooling; may show indoor temperature as -- or very high/low.

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

• Power cycle the thermostat: Turn off the furnace/air handler breaker for 2 minutes, then restore power. If E1 clears temporarily but returns, the sensor is likely failing.
• Check for environmental issues: Make sure the thermostat is not exposed to direct sunlight, drafts from supply registers, or heat sources (lamps, TVs). While this won't usually cause E1, extreme conditions can push a marginal sensor over its limits.
• Inspect thermostat internals (advanced DIY): Remove the thermostat from its sub‑base and place it on a clean table. If the design allows, gently open the housing. Look for a small bead or epoxy‑encapsulated thermistor near the air inlet slots. Check for broken leads or a loose plug‑in connector. Reseat any connectors carefully.
• Check for contamination: Dust or moisture on the sensor area can cause corrosion. Lightly blow out dust with dry compressed air. Do not use liquid cleaners.
• Factory reset: From the thermostat settings menu, perform a factory reset (consult the Payne smart thermostat manual). Re‑enter system configuration. If E1 persists, the sensor is likely defective.
• Replace thermostat: The internal thermistor is usually not field‑replaceable. If the sensor is open/shorted, replace the Payne smart thermostat assembly.