Furnace indicates a condensate or drain fault; the unit may shut down during operation, especially in cold weather.

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

• Locate condensate trap: On Sime condensing furnaces, the trap is usually a plastic assembly at the bottom of the heat exchanger or flue collector.
• Clean trap: Place a container underneath, remove the trap, and flush it with clean water to remove sludge. Check for any internal baffles and clean thoroughly.
• Inspect drain line: Ensure the plastic condensate pipe slopes continuously downward to the drain and is not kinked or blocked. Clear any visible obstructions.
• Defrost external pipe: In freezing conditions, gently warm the external section of the condensate pipe with warm (not boiling) water or a warm cloth. Do not use open flames.
• Reassemble and test: Refit the trap, ensuring seals are correctly seated. Restore power and run the furnace, checking for leaks and proper drainage.