Furnace attempts to start but locks out with E03, fan may run briefly, no ignition, often accompanied by a flue or air pressure warning icon.

Safety: Disconnect electrical power. Do not operate the Viadrus furnace with a suspected flue blockage; risk of carbon monoxide.

• Inspect flue outlet: From outside, check the flue terminal for ice, bird nests, leaves, or debris. Clear any obstruction. Ensure the terminal clearances meet local codes.
• Check fan operation: Remove the front cover and observe the flue gas fan (exhaust fan). With power restored briefly, call for heat and see if the fan starts. If the fan hums but does not spin, the motor may be seized or the capacitor (if fitted) may be faulty. Turn power off again before touching anything.
• Examine pressure switch and tubing: Locate the air pressure switch connected to the fan housing via silicone tubes. Check tubes for cracks, kinks, or water. Replace damaged tubing and ensure tight connections.
• Test pressure switch: With power off, disconnect the wires and measure continuity across the switch while gently applying suction to the tube. The switch should click and show continuity. If not, replace the air pressure switch.
• Fan replacement: If the fan does not run at all or is noisy and slow, the flue gas fan assembly should be replaced with a compatible Viadrus fan unit by a qualified technician.