High Lockout
Casting Thermistor Fault on Potterton Performa SL HE 9 2

Your boiler has detected a problem with a temperature sensor on the main internal heat exchanger, causing it to shut down 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

Automatic Lockout. Thermistor reading outside of limits or difference between flow and casting thermistor readings is outside of limits.

What causes this fault?

This fault occur when the sensor responsible for monitoring the temperature of the boiler's main heat exchanger sends an incorrect or out-of-range signal to the control board. The most common cause is a physical failure of the thermistor itself due to age, or potentially a loose connection in the wiring that prevents the boiler from safely monitoring internal heat levels.

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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