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 6 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.
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 ecoMAX Pro 28E manual (PDF)
- Vaillant ecoTEC exclusive manual (PDF)
- Vaillant ecoTEC Plus 938 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026