Error F.71

Safety precautions:

- Do not attempt gas-side repairs. If you smell gas, leave the property immediately and contact the gas emergency number.

- If you do any simple checks, first turn the boiler off at the user controls and isolate mains power at the external switch or fuse. Do not remove or touch internal components if you are not qualified.

- For anything beyond simple external checks call a Gas Safe registered engineer.

Initial checks a homeowner can do (safe, non-invasive):

1) Note the exact error (F.71) and any other codes or symptoms (no heating, no hot water, pump running, noises).

2) Try a boiler reset from the control panel or by switching the boiler off and on again at the external isolator. Wait a full minute before re-powering. If the fault does not reappear after reset and system works, monitor it — intermittent wiring or a sensor that is marginal may return.

3) Check visible system pressure on the boiler display. Very low water pressure can cause other faults; top up to the normal range if you know how and it is safe for your system.

4) Look for obvious signs of leaks, heavy condensation, or physical damage around the boiler and pipework. While you are not to open the boiler, a visible external pipe leak could affect sensor placement or wiring.

5) If the boiler is under a service contract or warranty, contact the service provider and report F.71; do not attempt internal repairs yourself.

Specific diagnostic and fix steps for a qualified engineer (do not attempt unless Gas Safe qualified):

1) Confirm and reproduce the fault by observing the PCB display and using the boiler’s diagnostic menu. Record any simultaneous errors (e.g., F.91 or others).

2) Isolate mains power and open the front casing following manufacturer guidance. Inspect the flow NTC sensor location (usually on the left side/outlet of the heat exchanger on EcoTEC models) and its plug connection. Check for a loose plug, broken latch, corroded pins, or water ingress.

3) Visually inspect the wiring harness from the NTC connector back to the PCB for chafing, damage or pinched wires. Wiggle test wiring with power off to identify intermittent breaks.

4) With power off, disconnect NTC plug and measure continuity and resistance of the sensor lead and sensor. With the sensor at room temperature expect a thermistor resistance in the ballpark of 10–12 kΩ at ~20 °C (confirm model-specific nominal from the manual). An open circuit (infinite resistance) or near zero (short) indicates sensor or wiring failure.

5) With the sensor disconnected, check the wiring connector at the PCB for correct seating and for any burnt or damaged pins. Check for a short to earth on the sensor cable.

6) If wiring and connectors are good but the sensor reads out of spec or constant, replace the flow NTC thermistor. Replacement normally involves fitting the correct NTC into the retaining clip on the flow pipe and plugging it back in securely.

7) If replacing the sensor does not fix the fault, check for wiring faults between the sensor and PCB, test the PCB input for correct voltages/signals, and check for coding or configuration issues as per Vaillant technical guidance. Repair or replace the harness or PCB as required.

8) After repair, restore power and run the boiler through a full call for heat and DHW to confirm the flow temperature now varies plausibly and the F.71 does not return. Run a full leak test and check system pressure.

Final notes and when to call a professional:

- If the fault persists after the basic, safe checks (reset, pressure, visible leaks), call a Gas Safe qualified engineer. The likely fixes involve testing and replacing NTC sensors, wiring harnesses or, rarely, PCB work — all tasks that require qualification and specialist tools.

- When you contact the engineer, tell them the model (EcoTEC Pro/Plus), the exact fault code F.71, what you have already tried (reset, checked pressure, any visible issues), and any pattern you noticed (intermittent, after power cut, after servicing). This speeds diagnosis and helps them bring the likely spare parts.