BoilerFaultCodesYour boiler has detected that a safety link or an external underfloor heating thermostat is missing or disconnected, causing the system to stop running for protection.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Terminals ST8-9 not shorted. Safety overheat thermostat for underfloor heating system at connector ST15 not fitted.
What causes this fault?
This fault occurs when a vital electrical safety loop has been broken, signals to the boiler that it might be overheating an underfloor heating system. It is often caused by a loose connection or a missing link wire that acts as a bridge to tell the boiler the system is safe to fire. If an underfloor heating thermostat is installed, it may have tripped or failed, forcing the boiler to shut down to prevent floor damage.
This fault is more common in autumn when homeowners turn on their underfloor heating for the first time after a long summer break, revealing issues with external thermostats.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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A boiler cover plan would cover faults like E5 — no call-out fees, no surprises.
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