Vaillant F.13 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.13 fault code mean?
The F.13 code tells you that the boiler's control board has detected a short circuit on the cylinder temperature sensor or warm start sensor circuit. These sensors use an NTC (Negative Temperature Coefficient) thermistor to feed live temperature data back to the PCB; when the signal collapses to a short, the boiler can no longer monitor cylinder or warm-start temperatures reliably and locks out to protect the system. F.13 most commonly appears on Vaillant combination boilers fitted with an actoSTOR unit or a warm start function, and it very frequently appears alongside fault code F.91 — if you see both codes, the cylinder charging sensor circuit is the prime suspect. There are no direct gas-safety implications, but the fault will disrupt hot water and heating operation until it is resolved.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Short-circuited NTC thermistor in the cylinder or warm start sensor Common
The thermistor itself develops an internal short — its resistance drops to near zero instead of the expected 10–12 kΩ at room temperature. This is the single most frequent root cause of F.13 and will trigger the fault even if all wiring is intact.
- Damaged or pinched sensor wiring loom Common
The cable connecting the cylinder or warm start sensor to the PCB can become chafed, kinked, or trapped, creating a short between conductors. This is especially likely on older installations where cables have been disturbed during servicing.
- Corroded or loose sensor connector plug Sometimes
The plug that mates the sensor lead to the PCB harness can suffer from moisture ingress, leading to corrosion that bridges contacts and mimics a short circuit. A loose plug can also cause an intermittent signal that the PCB interprets as a fault.
- PCB fault or damaged input circuit Rare
If the PCB's sensor input circuit has been damaged — by a voltage spike or previous water ingress, for example — it can falsely read a healthy sensor as a short. This is a less common cause but should be considered if a new sensor does not clear the fault.
How to fix it
- Reset the boiler once and observe whether F.13 returns DIY safe
Press and hold the reset button (usually the flame symbol with a line through it) for about three seconds. If F.13 clears and the boiler runs normally, monitor it over the next hour. If the code returns promptly, a reset will not cure the underlying fault and you should move on to the steps below rather than repeatedly resetting.
- Check that F.91 is also present and note any other active codes DIY safe
Use the boiler's diagnostic menu (on most ecoTEC models, press and hold the 'i' button) to scroll through any stored faults. If both F.13 and F.91 are showing, this strongly points to the cylinder charging sensor circuit rather than the warm start sensor. Note down everything you see to pass on to your engineer — it will speed up diagnosis.
- Visually inspect the sensor cables for obvious damage — engineer task Gas Safe engineer
A Gas Safe engineer will trace the wiring loom from the cylinder sensor and warm start sensor back to the PCB, looking for chafed insulation, trapped cables, or signs of moisture. This is not a job for a homeowner because it involves accessing internal boiler components.
- Test the NTC thermistor resistance — engineer task Gas Safe engineer
Using a calibrated multimeter, the engineer will disconnect the sensor plug and measure resistance across the thermistor terminals. A healthy NTC reads roughly 10–12 kΩ at room temperature (around 20 °C); a shorted thermistor will read close to 0 Ω. This test confirms whether the sensor or the wiring is at fault.
- Inspect and clean the sensor connector plug — engineer task Gas Safe engineer
The engineer will check the connector for corrosion, bent pins, or moisture. Corroded contacts can be cleaned with electrical contact cleaner; a damaged plug or socket will need replacing. Reconnecting a clean, well-seated plug sometimes resolves an intermittent F.13 on its own.
- Replace the faulty NTC sensor or cable harness — engineer task Gas Safe engineer
If the thermistor or its wiring is confirmed faulty, the engineer will fit a genuine Vaillant replacement part. The NTC sensor itself is inexpensive (typically £10–£30 for the part), making this a straightforward repair once the fault has been isolated. After replacement, the boiler should be fired and monitored to confirm F.13 does not recur.
- Investigate the PCB if the fault persists after sensor replacement — engineer task Gas Safe engineer
Should a new sensor fail to clear F.13, the engineer will inspect the PCB's sensor input circuit for burnt components or damage. PCB repair or replacement is a more involved job and the engineer may need to order parts, so allow extra time and budget if this route is necessary.
- Call a Gas Safe registered engineer Gas Safe engineer
All the diagnostic and repair work for F.13 must be carried out by a Gas Safe registered engineer. If you have not already done so, book one now — describe the F.13 (and F.91 if present) codes so they can bring the likely replacement parts. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- Vaillant cylinder / warm start NTC temperature sensor · from £18
- Sensor connector plug and terminal kit · from £12
- Vaillant wiring loom / cable harness (sensor section) · from £35
- Vaillant PCB (ecoTEC Plus/Pro) · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Vaillant F.13 myself?
Not really. Beyond resetting the boiler once and noting any additional fault codes, the repair involves accessing internal wiring and components and using a multimeter to test sensor resistance — all of which require a Gas Safe registered engineer. The part itself (an NTC thermistor) is inexpensive, but the diagnosis and fitting must be done professionally.
Why do F.13 and F.91 appear together on my Vaillant boiler?
F.91 is Vaillant's code for a fault specifically in the cylinder charging sensor circuit, while F.13 flags the short-circuit condition on that same circuit. When both codes appear together, they are pointing at the same problem from two angles: the cylinder charging NTC sensor, its wiring loom, or the connector plug. Telling your engineer both codes are present will help them go straight to the right component.
How much does it cost to repair Vaillant F.13 in the UK?
Most homeowners pay somewhere between £120 and £320 all in, depending on their region and whether the engineer needs one visit or two. The NTC sensor part itself is typically £10–£30, so the bulk of the cost is labour and the call-out fee. If the fault turns out to be a damaged PCB rather than the sensor, costs can be higher — a replacement Vaillant PCB can add £200–£300 to the bill — but this outcome is comparatively rare.
Will my Vaillant warranty cover an F.13 repair?
If your boiler is within its Vaillant parts-and-labour warranty period and has been serviced annually by a Gas Safe engineer, the repair should be covered or significantly subsidised. Contact Vaillant directly or the company that installed your boiler to check the warranty status before booking an independent engineer, as using an unauthorised repairer can sometimes affect warranty terms.