Boiler shows E03 and will not start, fan may run briefly then stop

Safety first: Turn off the boiler and isolate power. Do not operate the boiler if you suspect a flue or combustion issue; risk of carbon monoxide.

• Check external flue terminal: From outside, ensure the Sime concentric flue terminal is not blocked by leaves, plastic bags, snow, or bird nests. Clear any visible obstruction.
• Inspect condensate outlet: A blocked condensate trap or pipe can cause flue gases to overheat. Check the condensate pipe for kinks, freezing (in cold weather), or blockages. Thaw gently with warm (not boiling) water if frozen.
• Reset the boiler: After clearing any obvious blockage and allowing the boiler to cool, reset and observe. If E03 reappears, stop using the boiler.
• Check fan operation (engineer task): With the case removed, verify that the fan starts on demand and runs smoothly. Listen for grinding or squealing. Replace the fan if seized or noisy.
• Test flue thermostat (engineer task): On Sime boilers with a flue overheat stat, check continuity and mounting. Replace if open-circuit or if it trips repeatedly with a clear flue.
• Verify flue installation (engineer task): Confirm correct flue length, slope, and sealing according to Sime installation instructions. Incorrect flue configuration must be rectified immediately.