D.17 Low
Your boiler is switching from controlling the water temperature based on the outgoing flow to the returning water because it has detected a more stable reading that way.
Technical description: Heating glow return regulation
Typical repair cost: £90-250 (including labour)
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Protect yourself from future repair bills
A boiler cover plan would cover faults like D.17 — no call-out fees, no surprises.
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Parts commonly replaced
- NTC Thermistor (Flow/Return)
- Wiring harness
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