Medium Lockout
E321 on Potterton Promax HE

Your boiler's temperature sensor for hot water has stopped communicating correctly, which means you may have inconsistent or no hot water at your taps.

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 thermistor fault

What causes this fault?

This fault occurs when the temperature sensor responsible for your hot water fails to send a clear signal to the boiler's main control board. The most common reason is that the sensor has physically degraded over time or developed an electrical short, meaning the boiler can no longer tell how hot the water is. On some occasions, the issue may also be caused by loose or damaged wiring connecting the sensor to the rest of the system.

Engineer required

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

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