How to Fix Vaillant F71 Fault Code (Flow Sensor Detached) UK
Vaillant F71 means flow sensor detached. UK diagnosis, parts to order, reset sequence.
What F71 means
Vaillant F71 indicates flow sensor detached. The boiler will lock out and refuse to fire until the fault is cleared.
Do not simply reset — F71 is stored in the Vaillant error log and will re-trigger within minutes if the underlying part has failed.
Diagnostic sequence
Enter service mode on the Vaillant boiler and read the flow/return/DHW temperatures, fan PWM, ionisation current and pump speed.
Cross-reference against the Vaillant technical manual — F71 typically points to two or three candidate parts.
- Check wiring harness continuity to the suspect sensor before condemning it.
- Isolate at the fused spur, discharge and re-inspect connectors.
- Measure resistance / voltage at the PCB plug per Vaillant test procedure.
Parts to have on the van
Carry a set of NTC sensors, an ignition electrode kit and a fan for Vaillant — these three parts clear >70% of F71 call-outs.
Recommission
After the fix, run the Vaillant boiler through a full flow test, gas rate, and combustion analysis. Log to the Benchmark book to preserve manufacturer warranty.
Frequently asked questions
- Can I reset F71 without changing parts?
- Only if the fault has cleared (e.g. low pressure repressurised). Persistent F71 means a hardware failure.
- Which is the most common cause of Vaillant F71?
- On Vaillant boilers, F71 most often points to the sensor or actuator matching its meaning (flow sensor detached); check wiring and connector integrity first.
- Is a Gas Safe engineer required?
- Yes — any works past the boiler case seal require a Gas Safe registered engineer under UK law.
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