Saunier Duval F.71 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.71 fault code mean?
The F.71 code appears when your Saunier Duval boiler detects that the flow temperature sensor (NTC) is reporting a fixed, unchanging value instead of the live temperature readings the boiler expects. Because the boiler relies on this sensor to monitor the temperature of water leaving the heat exchanger, a stuck or static signal causes the boiler to shut down as a safety precaution. The most common reasons for this are that the sensor has slipped out of position on the flow pipe, or that the sensor itself has failed electrically. In rarer cases, a wiring fault or a problem with the PCB can produce the same symptom. Note that the neighbouring code F.70 relates to an invalid Device Specific Number (DSN) — typically triggered when a display and PCB are replaced simultaneously without the DSN being correctly configured — so if your display is alternating between F.70 and F.71 after recent board work, that distinction is worth flagging to your engineer.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Flow NTC sensor out of position Common
The flow temperature sensor must be firmly clipped onto the central heating flow pipe to take accurate readings. If it has worked loose or been dislodged, it will sense the surrounding air temperature instead of the water temperature, producing a suspiciously flat reading that triggers F.71. This is the first thing an engineer will check, as repositioning the sensor costs very little.
- Defective flow NTC sensor Common
NTC sensors degrade over time. The resistive element inside can fail in a way that outputs a constant voltage rather than one that varies with temperature, which is exactly the signal pattern that triggers this code. An engineer will measure the sensor's resistance across its operating temperature range to confirm failure before replacing it.
- Damaged or loose wiring to the sensor Sometimes
The wiring loom between the flow NTC sensor and the PCB can suffer from corrosion, chafing, or loose connector pins — particularly in older boilers or after recent servicing. A poor connection can mimic a failed sensor by delivering an unchanging signal to the control board.
- PCB fault Rare
If the sensor and all its wiring check out correctly, the PCB itself may be misreading or unable to process the sensor signal. PCB faults are the least common cause of F.71 but must be considered once everything else has been eliminated. Replacement is a significant job and cost.
How to fix it
- Check that your gas supply is working DIY safe
Before assuming the boiler is at fault, confirm that other gas appliances in your home — such as a hob or gas fire — are working normally. If there is no gas at all, contact your gas supplier rather than your boiler engineer.
- Reset the boiler once or twice DIY safe
Locate the reset button on your boiler's control panel (consult your user manual if unsure). Press and hold it as directed. The boiler display will flash during the reset attempt. If the boiler fires up and runs normally, monitor it over the next hour or so. If F.71 returns promptly, do not keep resetting — repeated resets will not resolve a sensor or wiring fault and may mask useful diagnostic information for the engineer.
- Check your system pressure DIY safe
Glance at the pressure gauge on the boiler. For most Saunier Duval models the normal operating range is 1.0–1.5 bar when cold. If pressure has dropped below 0.8 bar, top it up via the filling loop. Very low pressure can occasionally trigger secondary fault codes, though it is unlikely to be the root cause of F.71.
- Book a Gas Safe registered engineer to inspect the flow NTC sensor Gas Safe engineer
An engineer will start by checking whether the flow NTC sensor is correctly seated on the flow pipe. If it has moved, they will reposition and secure it, then run a test cycle to see if F.71 clears. This is the quickest and cheapest possible outcome.
- Allow the engineer to test and replace the flow NTC sensor if required Gas Safe engineer
Using a multimeter, the engineer will measure the sensor's resistance at a known temperature. If the readings are out of specification or static, the sensor needs replacing. Note that Saunier Duval is no longer actively trading in the UK (it is part of the Vaillant Group), so confirm part availability with your engineer before proceeding — some sensors are sourced through Vaillant's parts network.
- Allow the engineer to inspect the wiring loom and connectors Gas Safe engineer
If the sensor tests within specification, the engineer will trace the wiring from the sensor back to the PCB, checking for corroded pins, chafed cables, or intermittent connections. A wiring repair is usually straightforward and far cheaper than a board replacement.
- Allow the engineer to assess the PCB if all other checks pass Gas Safe engineer
Should the sensor and wiring both prove sound, the PCB is the remaining suspect. The engineer may be able to carry out a bench test or swap the board for a known-good unit to confirm. Given the cost of a PCB replacement on a boiler that may already be ageing, ask your engineer for an honest opinion on whether repair or full boiler replacement is the better investment at this stage.
Parts you may need
- Flow NTC temperature sensor · from £35
- Sensor wiring loom / harness · from £45
- PCB (control board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F.71 fault myself?
The homeowner checks — confirming your gas supply is on, resetting the boiler once or twice, and ensuring system pressure is in range — are safe to carry out yourself. Beyond that, diagnosing and replacing the flow NTC sensor or inspecting internal wiring requires opening the boiler casing, which must only be done by a Gas Safe registered engineer. There is no safe DIY shortcut for this fault.
How much does it cost to fix F.71 on a Saunier Duval boiler?
Most people with this fault pay between £150 and £350. If the sensor has simply shifted out of position, the bill may be towards the lower end once you include the call-out and labour. A new NTC sensor with fitting typically falls in the middle of that range. If the PCB turns out to be the cause, costs rise considerably — board replacement can reach £450–£600 or more — in which case it is worth weighing the repair cost against the age of your boiler.
Saunier Duval parts are hard to find — does that make this repair more expensive?
It can do. Saunier Duval no longer actively trades in the UK as an independent brand; parts are generally sourced through the Vaillant Group network. Most experienced engineers can access them, but lead times can sometimes be longer than for other brands. Always ask your engineer to confirm part availability before committing to a repair, particularly if your boiler is more than eight to ten years old, as some older model components can be difficult to obtain.
My boiler is over ten years old and showing F.71 — should I repair or replace it?
A sensor replacement on a relatively modern boiler is usually worthwhile. However, if your boiler is older than ten years and the engineer finds the PCB is also at fault, the combined repair cost can easily exceed half the price of a new boiler. At that point, upgrading to a new A-rated condensing boiler is often the smarter investment — modern models are significantly more efficient and could reduce your annual heating bills noticeably. Ask your engineer for a like-for-like comparison before deciding.