Medium Lockout
E321 on Baxi 600 Combi (range)

Your boiler has a faulty temperature sensor responsible for tracking the heat of your tap water, which usually results in inconsistent or no hot water.

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

Hot water negative temperature coefficient (NTC) fault.

What causes this fault?

This fault occurs when the sensor responsible for measuring the temperature of your hot water fails or sends an incorrect signal to the main control board. The most common reason is that the internal components of the sensor have degraded over time or developed a short circuit, meaning the boiler can no longer safely determine how hot the water is. Occasionally, this can also be caused by loose or damaged wiring connecting the sensor to the boiler's computer.

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 fault codes

E10View →E110View →

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 →