Boiler shows E03 and does not start, fan may not run or runs briefly then stops.

Safety: Switch off power and do not operate the boiler if you suspect a flue blockage. Risk of carbon monoxide.

• Check flue terminal: From outside, inspect the Thermona flue terminal for obstructions (ice, leaves, nests). Clear any visible blockage without dismantling the flue.
• Reset the boiler: After clearing the terminal, restore power and reset the boiler. Listen for the fan starting. If E03 returns, further checks are needed.
• Inspect air intake path: Ensure no objects are blocking the air intake around the boiler casing or flue connection.
• Pressure switch tubing (engineer level): Remove the front cover. Inspect silicone tubes between the fan venturi and the air pressure switch for splits, kinks, or water ingress. Replace damaged tubing and ensure correct routing.
• Air pressure switch test (engineer level): With a multimeter, check the switch changes state when the fan runs. If not, replace the Thermona air pressure switch with the correct model.
• Fan operation: If the fan does not start or is noisy, check its connector and supply from the PCB. A seized or slow fan can prevent the pressure switch from closing.

Flue and combustion components must be serviced by a qualified Thermona technician.