High Lockout
THERMISTOR on Potterton Promax FSB 30HE

Your boiler's internal temperature sensor has stopped sending a reliable signal, which means the system cannot safely monitor how hot the water is getting.

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

Lights: On * On Off. Possible causes: Thermistor resistance out of range (0.5kΩ - 20kΩ) or wiring fault between PCB and thermistor.

What causes this fault?

This fault occurs when the boiler's internal thermometer, called a thermistor, fails to measure the water temperature correctly. The most common reason is that the sensor has physically degraded over time or developed an electrical short, though it can also be caused by loose or corroded wiring to the main control board.

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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