Boiler intermittently shows F.19, especially when outdoor reset is enabled

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

• Locate outdoor sensor: Typically mounted on a north or northwest exterior wall, away from direct sun and other heat sources.
• Inspect sensor and cable: Check for physical damage, loose mounting, or water intrusion. Ensure the cable is intact from sensor to boiler control.
• Check connections: At the control board, verify the outdoor sensor wires are landed on the correct terminals and are tight.
• Reset and compare: Restore power and compare the displayed outdoor temperature to an actual thermometer. Large discrepancies indicate a bad sensor.
• Replace sensor: Install a new Lochinvar outdoor sensor if faulty, using UV-resistant cable and proper routing.

Professional help: If wiring and sensor are good but F.19 persists, the control board input may be defective.