The Vaillant F06 fault code indicates a problem with the flow temperature sensor (also called the flow NTC thermistor). When this sensor fails, is out of range, or its wiring is compromised, the boiler can’t reliably read the outgoing water temperature and will lock out for safety. On many Vaillant models, F06 specifically points to a faulty flow NTC rather than a return sensor issue.
| Likely Cause | What’s Happening | Who Should Act |
|---|---|---|
| Faulty flow NTC thermistor | The sensor reading is open circuit, shorted, or out of expected range | Gas Safe engineer to test/replace the thermistor |
| Loose/damaged wiring or plug | Sensor plug or loom is loose, corroded, or damaged | Engineer to re-seat, repair or replace wiring/connector |
| Moisture or corrosion at terminals | Damp connections interrupt or distort the sensor signal | Engineer to dry/clean or replace affected parts; check for leaks |
| PCB reading issue | Control board can’t read the sensor correctly | Engineer to test PCB input and replace if defective |
Confirm pressure: With the system cold, most Vaillant boilers should sit around 1.0–1.5 bar.
Look for leaks: Any damp around the boiler or nearby pipework should be addressed quickly.
One reset only: After the checks above, try one user reset. If F06 returns, stop and call a Gas Safe engineer.
Do not remove the boiler casing or probe wiring – that’s illegal unless you’re Gas Safe registered.
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| Engineer Step | Detail | Outcome |
|---|---|---|
| Sensor test | Measure flow NTC resistance vs. temperature; compare with spec | Confirms sensor state (ok/failed/out of range) |
| Wiring & connectors | Continuity checks, inspect plugs for corrosion/mechanical damage | Restores a clean, reliable signal path |
| Moisture/leak assessment | Identify any water ingress causing terminal corrosion | Prevents recurring sensor errors |
| PCB input check | Verify the board reads the NTC correctly under load | Rules in/out control board faults |
| Functional run test | Reassemble, reset, heat demand test; verify stable temps | Confirms the fault is cleared |
| Repair Item | Indicative Cost (UK) |
|---|---|
| Diagnostic visit (Gas Safe) | £70 – £120 |
| Flow NTC thermistor replacement | £90 – £150 |
| Wiring/connector repair | £100 – £180 |
| PCB replacement (if required) | £250 – £450 |
Prices vary by model, access, parts and region. We’ll confirm costs up-front.
In plain terms, Vaillant F06 means the boiler can’t trust the outgoing (flow) temperature reading. Without a reliable temperature, it won’t modulate the flame safely – so it locks out. This is different from return-side issues (often F01) and can show up after water ingress, accidental cable disturbance, or simply a sensor reaching end-of-life.
Hot then sudden shutdowns as control can’t verify temperature
Erratic temperature behaviour on radiators or hot water
Repeated lock-outs after resets
History of leaks/condensation near the boiler or pipework
If these line up with your situation, book a Gas Safe engineer promptly to prevent further disruption.
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If the boiler is otherwise healthy, an NTC replacement or small wiring fix is quick and cost-effective. If your boiler is ageing and you’re also seeing other faults (e.g., PCB errors, pump issues), discuss the economics of a new boiler installation – modern appliances are more efficient, quieter, and carry long warranties when installed by accredited companies.
Most manufacturers require annual servicing for the warranty to remain valid. Servicing checks sensors, wiring integrity and general safety – exactly the areas that help prevent F06. Consider a home care plan for predictable cost and priority response.
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What does Vaillant F06 mean?
A flow temperature sensor (flow NTC) fault — the boiler can’t read the outgoing water temperature reliably, so it locks out.
Can I fix Vaillant F06 myself?
No. You can check pressure, look for leaks, and try one reset — internal diagnostics and parts replacement are Gas Safe only.
Is F06 dangerous?
The lock-out is a safety response. Don’t bypass it – have an engineer investigate promptly.
What’s the quickest fix?
If the sensor has failed, replacing the flow NTC is typically fast and inexpensive. If the wiring or PCB is at fault, your engineer will advise.
How do I stop F06 happening again?
Keep up with annual servicing, fix leaks quickly, and avoid disturbing wiring inside the boiler.
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.
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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