Boiler intermittently shuts down with “Err 9” during operation, especially on cold days

Safety first: Turn off power before working on low-voltage wiring.

• Locate outdoor sensor: Find the small sensor housing on an exterior north or northwest wall, away from direct sun and exhaust vents. Confirm it is securely mounted.
• Inspect cable: Follow the cable back to the boiler. Look for cuts, staples through the wire, or UV damage. Repair or replace damaged sections.
• Check placement: If the sensor is in direct sun or near a dryer or boiler vent, relocate it to a shaded, representative location.
• Verify connections: At the boiler control, ensure the outdoor sensor wires are landed on the correct terminals and are tight.
• Replace sensor if needed: If the displayed outdoor temperature is obviously wrong or Err 9 persists, replace the outdoor sensor with the correct Burnham part.
• Test reset function: Restore power and confirm that the boiler now modulates based on a realistic outdoor temperature without error codes.