High Lockout
Overheat Stat neons flashing on Potterton Performa System HE

Your boiler has switched itself off because it has reached a temperature that is too high, often caused by a circulation problem or a blockage.

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

Overheat thermostat operated or faulty (Section J)

What causes this fault?

This fault occurs when the water inside the boiler gets too hot, triggering a safety switch to prevent damage. The most common reasons are a pump failure that stops water from moving or a buildup of debris like sludge that blocks the heat exchanger. It can also happen if there is trapped air in the system preventing proper circulation.

This issue often appears when the heating is first turned on after summer, as the internal pump can seize up after long periods of inactivity.

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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