D.9 Low
Your boiler is receiving a specific instruction from an external controller or thermostat telling it exactly what temperature to heat the water to.
Technical description: Flow target temperature from external analogue regulator terminal
Typical repair cost: £0-80 (including labour)
DIY fix possible
Check if your external room thermostat or smart home hub is set to a very low temperatureEnsure your external controls have power and are connected to your Wi-Fi if applicableIncrease the temperature setting on your external thermostat to see if the boiler fires up
Protect yourself from future repair bills
A boiler cover plan would cover faults like D.9 — no call-out fees, no surprises.
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Parts commonly replaced
- External thermostat batteries
- External heating controller
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