D.4 Low
Your boiler is currently showing the live temperature reading of the sensor responsible for keeping your hot water pre-heated.
Technical description: Current temperature for warm stat sensor
Typical repair cost: £0 (including labour)
DIY fix possible
No action required; this is an information code rather than a fault.Wait for the pre-heat cycle to finish and the display to return to normal.If you wish to stop this occurring, find the 'Comfort' mode setting and turn it off.
Protect yourself from future repair bills
A boiler cover plan would cover faults like D.4 — no call-out fees, no surprises.
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