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Vaillant F13 fault code: what it means and what to do
An F13 on your Vaillant points to a fault with the hot-water cylinder or warm-start temperature sensor — an internal electrical fault that needs a Gas Safe registered engineer. Here is what it means, why it happens, and what a fix typically costs in 2026.
Quick answer
F13 on a Vaillant boiler means a short circuit on the warm-start or hot-water cylinder temperature sensor (an NTC sensor). The boiler can no longer read the cylinder temperature reliably, so it flags the fault and may limit hot water. On some systems F13 appears alongside F91.
This is an internal sensor and wiring fault, so there is no safe DIY repair. You can try a single reset, but if F13 returns you should book a Gas Safe registered engineer to test and replace the sensor or its wiring.
What does F13 mean on a Vaillant boiler?
The F13 fault code on a Vaillant boiler indicates a short circuit on the warm-start sensor or the hot-water cylinder temperature sensor. These are NTC (negative temperature coefficient) sensors — small thermistors whose electrical resistance changes with temperature, letting the boiler measure how hot the stored water is.
When the boiler detects that this sensor's circuit has shorted, it can no longer trust the temperature reading. As a safety measure it logs F13 and may restrict or stop hot-water production.
On some Vaillant systems — particularly those with a separate storage cylinder or an actoSTOR unit — F13 is often shown in combination with F91, which points more specifically at the storage-tank sensor side of the circuit.
Because the exact wording and which sensor is involved can vary slightly between Vaillant ranges (ecoTEC, ecoFIT and cylinder-based systems), the safest reading of F13 is simply: a cylinder/warm-start temperature sensor circuit has failed and needs testing. It is an electrical fault inside the appliance, not something you have done wrong.
F13 is an engineer-only fault. It sits on the boiler's internal sensor wiring, so there are no homeowner-safe repair steps. Never open the boiler casing or touch any gas, flue or electrical parts — only a Gas Safe registered engineer may do that.
Common causes of F13
- A faulty NTC sensor — the cylinder or warm-start thermistor has failed or shorted internally.
- Damaged or chafed wiring in the cable harness running to the sensor.
- Loose, corroded or dislodged connectors at the sensor or on the PCB.
- A short to the boiler casing (a "mass short") caused by a trapped or pinched cable.
- Moisture ingress affecting the sensor connection.
- A PCB fault — less common, but the control board can misread the circuit even when the sensor itself is sound.
What you can safely check
Because F13 is an internal sensor fault, the homeowner-safe list is short. Do only these universal checks, then stop:
- Try one reset. Use the boiler's reset button once to see whether F13 was a momentary glitch. If you are unsure how, see our guide on how to reset your boiler. If the code returns, do not keep resetting — repeated lockouts can mask the real fault.
- Check the gas supply is on. Make sure other gas appliances work and the gas meter isn't off.
- Glance at the pressure gauge. F13 isn't a pressure fault, but it is worth knowing your normal reading — see what your boiler pressure should be.
That is the limit of safe DIY for F13. There is nothing further you can adjust without opening the appliance, which is illegal for an unregistered person.
When to call a Gas Safe registered engineer
If F13 does not clear after a single reset, book an engineer. Working on the sensor, its wiring or the PCB means accessing the sealed and electrical parts of the boiler, so it must be done by someone on the Gas Safe Register. It is both illegal and dangerous for an unregistered person to attempt this.
A competent engineer will typically:
- Read the fault and any paired codes (such as F91) from the boiler's memory.
- Measure the sensor's resistance and compare it against Vaillant's reference values.
- Inspect the cable harness, plugs and connectors for damage, chafing or corrosion.
- Replace the NTC sensor, wiring or connectors as needed.
- Test the PCB if the sensor and wiring check out but F13 persists.
If you ever smell gas or suspect a leak, leave the property and call the National Gas Emergency line on 0800 111 999. F13 itself is not a gas-escape fault, but that number is the right call in any gas emergency.
Typical Vaillant F13 repair cost
F13 is usually an affordable fix because the sensor itself is an inexpensive part — most of the bill is the engineer's time and diagnosis. These are indicative 2026 UK figures; your final price depends on your location, the boiler model and whether wiring or the PCB is involved.
| Job | Indicative 2026 cost |
|---|---|
| Diagnostic / call-out | £60 - £120 |
| Cylinder / warm-start NTC sensor (part) | £5 - £30 |
| Sensor replacement (parts + labour) | £120 - £200 |
| Wiring harness / connector repair | £90 - £180 |
| PCB replacement (if required) | £300 - £500+ |
For wider context on what heating jobs cost, see our guide to boiler repair costs.
Related Vaillant codes
If you are working through your Vaillant's display, these sibling guides cover other common codes:
- Vaillant F22 — low water pressure.
- Vaillant F28 — ignition failure on start-up.
- Vaillant F75 — pump / pressure-sensor fault.
You can also browse every code on our Vaillant fault codes hub.
Will boiler cover pay for an F13 repair?
Often it will.
A faulty cylinder sensor is a standard breakdown repair, and many boiler-cover policies include parts and labour for this kind of internal fault — but cover varies by policy and is typically subject to your excess and the usual exclusions for pre-existing faults or boilers deemed beyond economical repair.
Because the part is cheap and the labour is modest, an F13 alone may cost less than your policy excess, so it is worth checking your own policy wording and the figures before you claim. This is general information, not financial advice — always check the specific terms of your policy.
If you are weighing up a policy, our guides on what boiler cover includes and whether boiler cover is worth it walk through the trade-offs honestly.
Is the Vaillant F13 fault dangerous?
F13 is not in itself a gas-safety emergency — it is an electrical sensor fault that usually just limits or stops hot water. However, you should never try to fix it yourself, as that means opening the boiler. If you ever smell gas, call the National Gas Emergency line on 0800 111 999.
Can I fix F13 myself?
No. The sensor, its wiring and the PCB are inside the sealed boiler, and only a Gas Safe registered engineer may work on them. The only safe step you can take is a single reset; if F13 returns, book an engineer.
How much does it cost to repair F13?
Expect roughly £120 to £200 for a sensor replacement including parts and labour in 2026, as the NTC sensor itself is cheap (often only a few pounds to around £30). Costs rise if wiring or the PCB needs work.
Will F13 clear after a reset?
Sometimes a single reset clears a momentary glitch. But if the sensor or wiring is genuinely faulty, F13 will return — and repeated resetting won't fix it, so call an engineer instead of resetting again and again.
Why does F13 sometimes show with F91?
On Vaillant systems with a separate storage cylinder or actoSTOR unit, F13 paired with F91 points specifically at the storage-tank temperature sensor circuit. An engineer will read both codes to pinpoint which sensor and wiring are at fault.
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Compare boiler coverThis article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs are indicative UK guides for 2026.