Your boiler is displaying the temperature it is currently aiming for rather than a fault, which is a normal status message and not a breakdown.
This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.
Technical description: Flow temperature target value
What causes this fault?
This code appears when the boiler's internal computer calculates the specific temperature it needs to send to your radiators to meet your heating demands. It is physically triggered by the system's sensors and controls communicating the required heat level based on your thermostat settings. Because this is a standard status update rather than a failure, your boiler will continue to operate as normal.
DIY fix possible
Check if your thermostat or external controls are set higher than the current room temperatureWait for the boiler to complete its current heating cyclePress the 'i' button to exit the diagnostic information menu if you entered it accidentally
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A boiler cover plan would cover faults like D.40 — no call-out fees, no surprises.
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