Medium Lockout
F13 on Glow-worm Ultracom 2 CXI

Your boiler's hot water sensor has developed a technical fault, preventing the system from accurately measuring and heating your water.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Domestic hot water thermistor fault

What causes this fault?

This fault occurs when the sensor responsible for measuring your tap water temperature fails or provides an illogical electrical reading to the main control board. The most common reason is internal degradation of the thermistor over time, though it can also be caused by loose or corroded wiring connections between the sensor and the boiler’s brain.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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