Boiler shows error 31; unit may not start or may shut down shortly after starting.

Safety first: Isolate electrical power before removing covers. Water inside the boiler may be hot.

• Locate inlet thermistor: The inlet thermistor is typically clipped to or inserted into the cold water or return pipe entering the boiler.
• Check connections: Ensure the thermistor plug is fully seated on the sensor and at the control board. Look for corrosion or moisture on the pins.
• Inspect cable: Follow the thermistor cable for any signs of cuts, pinching, or heat damage.
• Reset and test: After reseating connectors, restore power and reset the boiler. If error 31 persists, the thermistor likely needs replacement.
• Thermistor replacement: Replacement involves draining the affected section of the system and swapping the sensor. This is usually straightforward but should be done carefully to avoid leaks.