BoilerFaultCodesYour heat pump has stopped working because a sensor that measures the water temperature returning from your home has developed a fault.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Outdoor Unit, Return Thermistor (Tw_In)
What causes this fault?
This fault occurs when the heat pump's control system loses the ability to accurately measure the temperature of the water returning from your radiators. This is usually caused by the internal electrical components of the sensor failing or the wiring becoming damaged, which prevents the unit from knowing how much heat is needed. Because the system can no longer safely monitor the heating cycle, it shuts down to prevent damage to the compressor.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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A boiler cover plan would cover faults like 9.1.12 — no call-out fees, no surprises.
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Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
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