F.12 High

Vaillant 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 28 Vaillant models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

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.

Typical repair cost £90-160 (including labour)

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026