BoilerFaultCodesVaillant F.1 Fault Code
Also written as F1 — same fault.
The Vaillant F.1 fault code means: your boiler has stopped working because it cannot correctly measure the temperature of the water returning to the unit due to a faulty sensor or connection.. It appears on 21 Vaillant models in our database.
Technical description: Return–NTC: NTC broken, NTC cable broken, Defective connection at NTC, Defective connection at electronics | Cause: Return–NTC cable defective/broken NTC faulty
What causes the Vaillant F.1 fault?
This fault occurs when the boiler's internal computer loses communication with the sensor that monitors the temperature of the water coming back from your radiators. The most common reason is that the sensor itself has failed due to age or electrical failure, though it can also be caused by a loose connection or damaged wiring. Because the boiler can no longer safely monitor heat levels, it shuts down as a protective measure.
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 F.1 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £90-250
- ecoMAX Repair: £90-180
- ecoMAX 613 Repair: £90-160
- ecoMAX 618 Repair: £90-160
- ecoMAX 622 Repair: £90-160
- ecoMAX 635 Repair: £90-160
- ecoMAX 824E Repair: £90-160
- ecoMAX 828E Repair: £90-160
- Ecomax Condensing 600/800 series Repair: £120-200
- ecoMAX Pro 28E Repair: £100-280
- ecoMAX System 612 Repair: £90-160
- ecoTEC Repair: £90-160
- ecoTEC 65kW Repair: £120-200
- ecoTEC Plus 615 Repair: £100-£220
- ecoTEC Plus 620 Repair: £100-£220
- ecoTEC Plus 625 Repair: £100-£220
- ecoTEC Plus 630 Repair: £100-£220
- ecoTEC Plus 635 Repair: £100-£220
- ecoTEC Plus 840 Repair: £100-£220
- ecoTEC Pro / ecoTEC Plus Repair: £120-220
- ecoTEC sustain Repair: £100-£220
Source
Vaillant F.1 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
- Vaillant ecoMAX manual (PDF)
- Vaillant ecoMAX System 612 manual (PDF)
- Vaillant ecoMAX 824E manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026