High Lockout
4.14 on Ideal Logic Max System

Your boiler is having trouble measuring the temperature of your hot water, which usually means it cannot heat water safely or effectively.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

DHW Thermistor Fault

What causes this fault?

This fault occurs when the sensor responsible for monitoring your hot water temperature has failed or is sending incorrect readings to the main control board. The most common reason is that the internal components of the sensor have degraded over time, though it can also be caused by loose or damaged electrical wiring. Because the boiler cannot accurately tell how hot the water is, it shuts down as a safety precaution to prevent overheating.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

F2View →L2View →

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on combis.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →