High Lockout
E20 on Potterton Promax

Your boiler is struggling to monitor its temperature correctly because a internal sensor is sending a faulty signal, preventing the heating from working properly.

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

Central heating thermistor sensor fault

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor that monitors water temperature. The most common reason is that the thermistor has physically degraded over time or suffered an electrical failure, meaning it can no longer tell the boiler if the water is safe to heat. In some cases, loose or damaged wiring between the sensor and the main control board can also trigger this specific error.

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

E28View →E50View →

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