Boiler attempts to start but shuts down with E03. Fan may run briefly. No ignition. Often occurs on room-sealed Viadrus gas boilers with flue fan.

Safety first: Turn off power and gas. Flue gas issues can lead to carbon monoxide hazards. If you suspect flue blockage or smell exhaust, do not operate the boiler until inspected by a professional.

Step-by-step checks:

• 1. Visual flue inspection: From outside, check that the flue terminal is not blocked by leaves, nests, snow, or debris. Ensure the terminal guard (if fitted) is clear.
• 2. Inspect fan and pressure tubes: Remove the boiler front cover. Locate the flue fan and the small silicone tubes running to the air pressure switch and venturi. Check for splits, kinks, water, or blockages. Clean or replace tubes as needed.
• 3. Check fan operation: With the cover back in place and power restored, call for heat and listen for the fan. If the fan does not start, check the fan connector and supply voltage. A non-spinning fan with correct voltage indicates a failed fan motor that must be replaced.
• 4. Test air pressure switch (basic): With power off, disconnect the wires from the pressure switch and check for loose spade terminals. Gently blow through the tubes to ensure they are clear. Do not adjust the switch calibration screw.
• 5. Reset and test: After cleaning tubes and confirming the flue terminal is clear, reset the boiler. If E03 persists, the pressure switch or fan assembly likely needs professional testing and replacement.

When to stop: If you cannot confirm a clear flue path or if the fan/pressure switch requires replacement, contact an authorised Viadrus service technician. Do not operate the boiler with suspected flue faults.