BoilerFaultCodesVaillant d.73 Fault Code
Also written as D73 — same fault.
The Vaillant d.73 fault code means: your boiler's pre-heat function for hot water isn't reaching the temperature it expects, which may lead to slower or inconsistent hot water delivery.. It appears on 27 Vaillant models in our database.
Technical description: Difference between warm start and hot-water outlet target value
What causes the Vaillant d.73 fault?
This fault occurs when the boiler struggles to keep its internal store of hot water at the correct temperature for the 'warm start' feature. The most common reasons are a buildup of limescale in the heat exchanger or a failing sensor that is giving the control board inaccurate temperature readings.
This issue is more noticeable in winter when the incoming mains water is much colder, forcing the boiler to work harder to maintain its pre-heat temperature.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Vaillant models that show the d.73 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £0-80
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825 Repair: £90-180
- ecoFIT Pure 830 Repair: £0
- ecoFIT Pure 835
- ecoMAX Repair: £90-250
- ecoMAX 613 Repair: £150-250
- ecoMAX 618 Repair: £150-250
- ecoMAX 622 Repair: £150-250
- ecoMAX 635 Repair: £150-250
- ecoMAX 824E Repair: £150-250
- ecoMAX 828E Repair: £150-250
- Ecomax Condensing 600/800 series Repair: £120-250
- ecoMAX System 612 Repair: £150-250
- ecoTEC Pro / ecoTEC Plus
- ecoTEC sustain
- Home Combi
- Thermocompact
- Turbomax 824/828 Repair: £120-250
- Turbomax Plus Repair: £120-220
- Turbomax Plus 824/828E
- Turbomax Plus 828 Repair: £80-150
- Turbomax Plus 828 E Repair: £100-200
- Turbomax Plus VUW 242E Repair: £100-200
- Turbomax Plus VUW 242EH
Source
Vaillant d.73 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoMAX manual (PDF)
- Vaillant ecoMAX System 612 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026