Boiler intermittently displays E110 and stops heating, often during long runs

Safety first: Turn off power and allow piping to cool before handling sensors.

• Step 1 – Locate return sensor: Find the return water temperature sensor on the return manifold or piping entering the boiler.
• Step 2 – Inspect mounting: Ensure the sensor is firmly attached to the pipe or well and has good thermal contact (use thermal paste if specified).
• Step 3 – Check wiring: Inspect the harness for damage and confirm the connector at the PCB is secure.
• Step 4 – Replace sensor: If E110 persists, replace the return thermistor with a Navien‑approved part.

Incorrect temperature readings can cause poor modulation and nuisance shutdowns, so address E110 promptly.