Boiler displays F.05 and shuts down, may occur intermittently during long runs

Safety first: Disconnect power before accessing the control compartment.

• Locate inlet sensor: On Lochinvar boilers, the inlet (return) water sensor is typically threaded into the return header or attached with a well/clip. Refer to your model's manual for exact location.
• Inspect wiring: Check the sensor harness from the sensor to the main control board. Look for pinched wires, corrosion at connectors, or loose plugs. Reseat the connector firmly.
• Check sensor mounting: Ensure the sensor is fully inserted into the well or threaded tightly with thermal paste if required. Poor contact can cause erratic readings.
• Reset control: Restore power and clear the fault. Monitor displayed inlet temperature; if it jumps erratically or reads unrealistic values (e.g., -40°F or 250°F), the sensor is likely defective.
• Replace sensor if needed: If you are comfortable, shut off and drain the boiler section as needed, then replace the inlet sensor with an OEM Lochinvar NTC sensor. Use thread sealant approved for hydronic use if threaded.

Professional help: If a new sensor does not resolve F.05, the control board analog input may be faulty and should be diagnosed by a technician.