Unit attempts to start but quickly shows error 03; fan may not run or runs briefly.

Safety: Disconnect power. Do not operate the heater with the flue system disassembled.

• Check flue terminal: Go outside and inspect the Immergas concentric flue terminal. Remove any visible obstructions such as leaves, nests, or ice. Ensure the terminal is not covered or modified.
• Inspect air intake path: For room‑sealed models, ensure the air intake annulus of the concentric flue is clear. For open‑flue models, ensure the room has adequate ventilation and the chimney is not blocked.
• Check condensate trap: Locate the condensate trap at the base of the combustion chamber. Remove and clean it, flushing out sludge and debris. Refill with clean water before refitting.
• Verify fan operation: With the cover removed and power restored, briefly call for hot water and observe if the fan starts. If the fan does not run or is noisy and slow, the fan motor may be failing.
• Air pressure switch: Inspect the silicone tubes from the fan/venturi to the air pressure switch. Replace any cracked or water‑filled tubes. Ensure the switch wiring is secure.

Warning: Do not bypass the air pressure switch or operate the unit with a known flue fault. If error 03 persists after basic checks, contact an authorized technician.