Saunier Duval F10 Fault Code: Central Heating Return Thermistor Fault
The F10 code indicates that the boiler's central heating return thermistor (NTC sensor) has developed a fault. This small temperature sensor sits on the return pipework and continuously reports the temperature of water coming back into the boiler from the heating circuit. When the boiler's control board detects that the sensor's signal is missing, out of range, or short-circuited, it triggers F10 and shuts the boiler down to prevent unsafe operation.
General guidance only — not a substitute for professional advice. 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.
What does the Saunier Duval F10 fault code mean?
The F10 code indicates that the boiler's central heating return thermistor (NTC sensor) has developed a fault. This small temperature sensor sits on the return pipework and continuously reports the temperature of water coming back into the boiler from the heating circuit. When the boiler's control board detects that the sensor's signal is missing, out of range, or short-circuited, it triggers F10 and shuts the boiler down to prevent unsafe operation.
Common causes
- Faulty or degraded return NTC thermistor Common
Over time the NTC sensor element itself can fail due to age, corrosion, or heat cycling. A functioning return thermistor should measure approximately 10 kΩ resistance at 25 °C — a reading far outside this points directly to sensor failure and is the single most common cause of F10.
- Loose or disconnected sensor wiring Common
The thermistor connects to the PCB via a short wiring harness and push-fit connector. Vibration or previous servicing can cause the connector to work loose, breaking the signal circuit and triggering the fault even though the sensor itself is still in good condition.
- Short circuit in the sensor cable or connector Sometimes
A pinched, chafed, or heat-damaged cable can cause the wiring to short to earth or to the boiler casing. The PCB interprets this as an impossible sensor reading and raises F10. Visual inspection of the cable run often reveals the damage point.
- PCB fault misreading a good sensor Rare
In a small number of cases the return thermistor and its wiring test as perfectly healthy. Here, the PCB input channel responsible for processing the sensor signal has failed. This is relatively uncommon but should be considered once the sensor and wiring have been ruled out.
How to fix it
- Check whether this is a one-off event and reset the boiler DIY safe
Press and hold the reset button for roughly three seconds until the boiler attempts to restart. Allow it a couple of minutes to complete its start-up sequence. If it runs normally, monitor over the next day or two. Avoid resetting more than two or three times in quick succession — repeated lockouts without a successful start mean the underlying fault needs investigation.
- Inspect the condensate pipe and system pressure DIY safe
While F10 points to the return thermistor, it is worth quickly ruling out any secondary issues. Check the pressure gauge shows between 1.0 and 1.5 bar; if it is below 1 bar, top it up via the filling loop. Also check the condensate pipe has not frozen during cold weather. Neither of these will cause F10 directly, but they can lead to additional fault codes if ignored.
- Have a Gas Safe engineer inspect the thermistor wiring and connector Gas Safe engineer
The engineer will power down the boiler safely and trace the return thermistor wiring harness from the sensor body to its connector on the PCB. They will check for loose pins, corrosion inside the plug, and any visible cable damage. Reconnecting a loose connector or cleaning a corroded contact can sometimes resolve the fault without a part replacement.
- Test the return thermistor resistance Gas Safe engineer
Using a multimeter, the engineer will measure the resistance across the thermistor terminals. At 25 °C the reading should be in the region of 10 kΩ. A reading that is significantly higher, lower, or open-circuit confirms the sensor has failed and needs replacing. This test takes only a few minutes and is definitive.
- Replace the return NTC thermistor Gas Safe engineer
If the sensor is confirmed faulty, the engineer will fit a replacement — Saunier Duval return NTC sensors are a relatively inexpensive, widely stocked part. After fitting, the boiler is reset and run through a full heating cycle to confirm the new sensor is reading correctly and the F10 code has cleared.
- Investigate the PCB if the fault persists after a new sensor Gas Safe engineer
If F10 returns despite a confirmed good sensor and sound wiring, attention turns to the boiler's PCB. The engineer may test the relevant input channel or arrange for a PCB replacement or repair. This is a less common but legitimate route when all other causes have been ruled out.
- Call a Gas Safe registered engineer if the fault does not clear on reset Gas Safe engineer
All internal boiler work — sensor replacement, wiring checks, and PCB investigation — must be carried out by a Gas Safe registered engineer. If resetting the boiler does not resolve the lockout, book an engineer visit promptly. Running the boiler in a fault state or attempting internal repairs without Gas Safe registration is unsafe and may invalidate your warranty or home insurance.
Parts you may need
- Central heating return NTC thermistor (e.g. Saunier Duval part S5739800) · from £22
- Thermistor wiring harness / connector loom · from £18
- Boiler PCB (if sensor input channel has failed) · from £195
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 £100–£220, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F10 fault myself?
The reset button is the only homeowner action worth trying. If the boiler fires up and stays running after a reset, monitor it closely. Any work inside the boiler — checking the sensor, testing resistance, or replacing the thermistor — must be done by a Gas Safe registered engineer. Working on boiler internals without Gas Safe registration is illegal and dangerous.
How much does it cost to fix an F10 fault on a Saunier Duval boiler?
For a straightforward return thermistor replacement during standard weekday hours, most homeowners pay somewhere between £100 and £220 all in. The sensor itself is inexpensive (typically under £30); the bulk of the cost is call-out and labour. Engineers in London and the South East tend to be at the top of that range. If the PCB turns out to be at fault, costs can rise considerably — a replacement PCB plus fitting can run to £350–£500 — but this scenario is uncommon.
Why does F10 appear on some Saunier Duval models but mean something slightly different?
Most Saunier Duval ranges (Thema Condens, Themaclassic, ISOTWIN Condens) use F10 to flag the central heating return thermistor fault, which is the manufacturer-confirmed primary meaning. On a small number of older or variant models the code has been associated with a pressure sensor issue instead. If you are unsure which applies to your boiler, check the fault code table in your specific model's user guide, or ask your engineer to cross-reference with the service manual for your serial number.
How long does a return thermistor replacement take?
Once an engineer is on site and has confirmed the diagnosis, swapping out a return NTC thermistor is usually a straightforward job — most take between 30 minutes and an hour including a test run. If the wiring harness also needs replacing or the PCB needs investigation, allow a little longer, but in the majority of cases the boiler is back up and running within a single visit.