Medium Lockout
LED2 flashing, LED3 on on Ideal Mini C28

Your boiler has detected a sensor problem that is preventing it from providing hot water correctly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Faulty DHW thermistor or wiring

What causes this fault?

This fault occurs when the sensor responsible for measuring the temperature of your hot water fails to provide a consistent reading to the boiler's control board. The most common reason is that the internal components of the sensor have degraded over time, though it can also be triggered by a loose or damaged connection in the electrical wiring. As a safety precaution, the boiler stops heating water to prevent it from becoming dangerously hot or staying stone cold.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

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 →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →