Furnace displays E03 and stops; often associated with flue or air pressure switch problems.

Safety first: Turn off power and gas. Do not operate the furnace with a suspected flue fault.

• Check flue terminal and intake: From outside, ensure the Sime flue terminal is clear of obstructions. Remove any debris carefully.
• Inspect pressure tubes: Open the furnace and locate the silicone tubes between the fan and air pressure switch. Check for water, kinks, or splits. Replace or clear as needed.
• Test fan operation: Restore power briefly and call for heat to see if the fan starts. If not, the fan or its power supply may be faulty.
• Replace air pressure switch if needed: If the fan runs and tubes are clear but E03 persists, the switch may be defective and should be replaced by a technician.