BoilerFaultCodesYour boiler has detected a problem with the sensor that monitors exhaust fumes, causing the unit to shut down for safety.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flue gas sensor S4 defective
What causes this fault?
This fault occurs when the boiler's internal safety system loses communication with the sensor that monitors the temperature of the exhaust fumes. It is usually caused by a physical failure of the sensor's internal components or loose electrical connections in the wiring harness. When the boiler cannot verify if the exhaust gases are at a safe temperature, it will stop running immediately to prevent potential heat damage.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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