BoilerFaultCodesVaillant d.43 Fault Code
Also written as D43 — same fault.
The Vaillant d.43 fault code means: your boiler is notifying you that it is using a specific calculated temperature setting to heat your home, typically when an outdoor weather sensor is connected.. It appears on 20 Vaillant models in our database.
Technical description: Heating curve
DIY fix possible
Check your external weather compensator or smart thermostat settingsAdjust the heating curve setting in the installer menu if the house is too cold or too warmConsult your thermostat manual to ensure the 'Heat Curve' matches your home's insulation level
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the d.43 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT pure Repair: £0-80
- ecoFIT Pure 412
- ecoFIT Pure 418 Repair: £80-140
- ecoFIT Pure 425 Repair: £0-120
- ecoFIT Pure 435 Repair: £0-90
- ecoFIT Pure 835 Repair: £0-90
- ecoTEC plus
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 65
- ecoTEC Plus 832
- ecoTEC Plus 836
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus System
- ecoTEC sustain
- Home System
Source
Vaillant d.43 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026