Seeing the Vaillant F10 fault code usually points to a flow temperature sensor (flow NTC) short-circuit or a shorted wiring/connector on that circuit. When the boiler can’t trust the outgoing water temperature reading, it locks out for safety.
This guide explains what Vaillant F10 means, common causes, safe homeowner checks, how an engineer will fix it, typical UK costs, and how to stop it returning.
In plain English: the boiler believes the flow temperature sensor is reading short-circuit / too low resistance (sometimes described as “short to earth”). Without a valid flow temperature, the boiler won’t fire or will immediately lock out. This is the flow (outgoing) pipe sensor, not the return sensor.
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| Likely Cause | What’s Happening | Recommended Action |
|---|---|---|
| Failed flow NTC (short-circuit) | Sensor resistance has dropped to an out-of-range “short” value | Gas Safe engineer to test/replace the flow NTC |
| Damaged/shorted wiring or plug | Pinched cable, rubbed insulation, or bridged pins causing a short | Repair/replace loom; re-seat and secure connectors |
| Moisture/corrosion at terminals | Water ingress leads to tracking and false short readings | Dry/clean terminals; fix the source of leaks or condensation |
| PCB input fault | Control board misreads the sensor even when it’s healthy | Engineer to verify PCB input; replace if out of spec |
System pressure: With the system cold, aim for ~1.0–1.5 bar (per your manual).
Look for leaks: Any damp below/around the boiler or pipework should be reported quickly.
One reset only: After external checks, try one reset from the user controls. If the Vaillant F10 fault code returns, stop and call a Gas Safe engineer.
Do not remove the boiler casing or probe wiring – internal work is Gas Safe only.
| Engineer Step | Detail | Outcome |
|---|---|---|
| Confirm sensor fault | Measure flow NTC resistance vs. temperature; compare to spec sheet | Confirms sensor state (OK/shorted) and rules out false flags |
| Check loom & connectors | Continuity and insulation checks; inspect for shorts, pinches, corrosion | Restores a clean, stable signal path |
| Moisture/leak rectification | Identify/stop water ingress; dry, clean and protect terminals | Prevents repeat “short” readings |
| PCB verification | Verify PCB input range and live readings under load | Determines if board replacement is required |
| Functional run test | Reassemble, reset, call for heat; verify stable modulation and temps | Proves the fault is fully cleared |
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| Repair Item | Indicative Cost (UK) |
|---|---|
| Diagnostic visit (Gas Safe) | £70 – £120 |
| Flow NTC sensor replacement | £90 – £150 |
| Wiring/connector repair | £100 – £180 |
| PCB replacement (if required) | £250 – £450 |
Prices vary by model, parts, access and region. We confirm costs up-front.
Immediate re-lock after a reset (sensor still reading short)
Erratic temperature behaviour followed by lock-out
A history of leaks/condensation near the boiler or within the case
Event log entries showing sensor/NTC faults alongside other Vaillant boiler fault codes
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If the appliance is otherwise healthy, replacing the flow NTC or repairing a shorted loom is a quick, low-cost fix. If the boiler is older and you’re also seeing board failures, pump issues or repeated sensor faults, it may be worth pricing a new boiler installation with modern controls and a long warranty.
If the appliance is otherwise healthy, replacing the flow NTC or repairing a shorted loom is a quick, low-cost fix. If the boiler is older and you’re also seeing board failures, pump issues or repeated sensor faults, it may be worth pricing a new boiler installation with modern controls and a long warranty.
To reduce the chance of the error returning:
Keep the boiler area dry and leak‑free
Use quality parts when repairs are needed
Consider a boiler cover plan for ongoing protection
Upgrade older sensors when advised
These steps help maintain accurate temperature control and prolong boiler lifespan.
What does Vaillant F10 mean?
A flow temperature sensor (flow NTC) short-circuit or a related wiring/PCB issue that causes the boiler to lock out.
Can I fix Vaillant F10 myself?
No. You can check pressure, look for visible leaks and try one reset – internal diagnostics are Gas Safe only.
Will a reset clear it?
A reset might clear a one-off glitch, but a true flow NTC short will typically re-appear until repaired.
Is it dangerous?
The lock-out is a safety response. Don’t bypass it; book a qualified engineer.
How do I stop it returning?
Fix any moisture sources, ensure sound wiring, and keep up with annual servicing.
For more information on other Vaillant fault codes, you can refer to the following:
Consider exploring the following services to maintain and enhance your heating system:
For more information on Vaillant Boilers you can refer to the following articles:
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