High Lockout
AL73 on Vokera Mynute 23E

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning from your heating system, causing it to stop running for safety.

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

Return thermistor fault

What causes this fault?

This fault occurs when the internal control board loses communication with the sensor that monitors water returning from your radiators. It is usually caused by the sensor's internal components wearing out over time or a loose connection in the internal wiring, preventing the boiler from safely regulating its temperature.

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.

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Related fault codes

AL71View →AL79View →

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