Boiler locks out with “Err 4” and the vent fan may run continuously or not at all

Safety first: Turn off power and gas. A venting fault can cause carbon monoxide buildup. If you suspect exhaust leakage, leave the area and call a professional.

• Inspect vent and intake terminations: Outside the building, check the PVC or metal vent and intake for ice, leaves, nests, or debris. Clear any obstructions and ensure screens are clean.
• Check condensate drain: For Burnham condensing boilers (e.g., Alpine, K2, Aspen), locate the condensate trap and tubing. Make sure the trap is not clogged with sludge and the drain line slopes downward and is not frozen or kinked.
• Verify inducer operation: With the front panel removed and power restored, call for heat and observe the combustion fan/inducer. It should start smoothly. If it hums, grinds, or does not spin, shut power off and inspect for foreign objects or seized bearings.
• Test air pressure switch (basic): Locate the round or rectangular pressure switch connected by silicone tubing to the fan or flue. Ensure tubing is not cracked, blocked, or full of water. Re-seat the tubing firmly.
• Reset and test: After clearing any visible issues, restore power and reset the control. If Err 4 returns, the pressure switch or inducer motor may need electrical testing and replacement by a technician.