The furnace shows E20; burner output is limited or shut down.

Safety first: Isolate power. Condensate is mildly acidic; wear gloves and eye protection.

• Inspect condensate pipe: Follow the plastic condensate pipe from the furnace to the drain. Check for kinks, sags, or external freezing in cold weather.
• Thaw frozen pipe: If frozen, gently warm the pipe with warm (not boiling) water or a heat pack. Do not use open flames.
• Clean condensate trap: Remove the condensate trap beneath the furnace, empty it, and rinse thoroughly to remove sludge and debris. Refit securely, ensuring seals are in place.
• Check fall to drain: Ensure the pipe slopes continuously downward to the drain to prevent standing water.
• Sensor check: On models with a condensate level sensor, inspect and clean the sensor. Replace if faulty.