Boiler locks out with E03; fan may not start or stops quickly; no ignition attempt

Safety: Turn off power and gas. Combustion air and flue issues can cause carbon monoxide hazards—do not operate the boiler with venting disassembled.

• Inspect venting: From the boiler to the termination, check NTI concentric or twin‑pipe vent for obstructions (ice, leaves, bird nests). Ensure terminations are not blocked by snow or debris.
• Check pressure tubing: Remove the silicone tubes from the air pressure switch and fan nipple. Look for water, kinks, or cracks. Blow gently through the tubes to clear condensate and reinstall securely.
• Test air pressure switch: With power off, disconnect the wires and check continuity across the switch while gently applying suction to the tube. It should click and change state. Replace if it does not operate reliably.
• Verify fan operation: Restore power and call for heat while observing the fan. If the fan does not start or is noisy and slow, the combustion fan assembly may be failing and should be replaced by a technician.