BoilerFaultCodesVaillant F.12 Fault Code
Also written as F12 — same fault.
The Vaillant F.12 fault code means: your boiler has lost the ability to measure the temperature of your hot water cylinder because of a faulty sensor or a wiring issue, meaning it cannot heat your water correctly.. It appears on 26 Vaillant models in our database.
Technical description: Short circuit: Cylinder temperature sensor. NTC sensor defective, short circuit in the cable harness, cable/housing
What causes the Vaillant F.12 fault?
This fault occurs when the electrical signal from the hot water temperature probe is interrupted, usually because the sensor has failed internally. The most common reason is that the sensor has developed a physical short circuit or the wiring connecting it to the main control board has become damaged or loose. Because the boiler can no longer tell how hot the water is, it stops heating the cylinder as a safety precaution.
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.12 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £150-£450
- ecoFIT pure 612
- ecoFIT pure 615
- ecoFIT pure 630
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoFIT sustain 825
- ecoFIT sustain 835
- ecoMAX Pro 28E Repair: £150-300
- ecoTEC
- ecoTEC exclusive 627
- ecoTEC plus 612
- ecoTEC plus 618
- ecoTEC Plus 620 Repair: £90-160
- ecoTEC plus 624
- ecoTEC Plus 625 Repair: £90-160
- ecoTEC Plus 630 Repair: £90-160
- ecoTEC Plus 635 Repair: £90-160
- ecoTEC plus 637
- ecoTEC Plus 826 Repair: £120-220
- ecoTEC Plus 832 Repair: £90-160
- ecoTEC Plus 840 Repair: £90-160
- ecoTEC Plus 938 Repair: £90-160
- ecoTEC plus aquaPLUS Repair: £90-160
- ecoTEC Pro / ecoTEC Plus
Source
Vaillant F.12 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 630 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
- Vaillant ecoFIT sustain 835 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026