Saunier Duval F.00 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.00 fault code mean?
The F.00 fault code (sometimes displayed as F0) appears on Saunier Duval boilers when the control board loses its signal from the flow temperature NTC thermistor. This sensor continuously monitors the temperature of water leaving the heat exchanger, feeding real-time data back to the PCB so the boiler can regulate the burner output. When the board detects no signal — or a signal so far outside the expected range that it cannot be trusted — it shuts the boiler down and displays F.00. Both central heating and hot water will be unavailable until the fault is resolved. The fault can point to the sensor itself, the wiring between the sensor and the PCB, or a loose plug connection at either end. A healthy flow NTC thermistor reads approximately 10 kΩ resistance at 25°C; as water temperature rises, resistance falls — so a reading that stays fixed or is wildly out of range confirms a faulty component.
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
- Failed or degraded flow NTC thermistor Common
Over time, the thermistor element inside the sensor deteriorates, causing it to produce readings that are erratic, completely absent, or stuck at one value. This is the single most common cause of F.00. Older boilers — particularly those over eight to ten years old — are especially prone, as the sensor is constantly exposed to hot water cycling.
- Loose or corroded plug connection at the sensor or PCB Common
The NTC sensor connects to the PCB via a plug-and-socket connector. Vibration over years of use, or moisture ingress into the boiler casing, can cause these connectors to work loose or develop corrosion on the pins. Even a slightly poor connection is enough to break the signal and trigger F.00.
- Damaged sensor wiring Sometimes
The cable running between the flow sensor and the PCB can become chafed, pinched, or heat-damaged if it runs close to the flue or other hot components. A break or short in this wiring will interrupt the signal in the same way as a failed sensor.
- PCB fault Rare
In a minority of cases, the control board itself is the problem — either the sensor input circuit has failed or the board is misinterpreting a perfectly good sensor signal. This is a less common cause and is usually only confirmed after the sensor and wiring have been checked and found to be in good condition.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button (refer to your boiler's handbook for the exact location — on most Saunier Duval models it is a dedicated button on the control panel). If the boiler fires up and runs normally, monitor it closely over the next hour. A one-off F.00 can occasionally be caused by a momentary signal glitch rather than a hardware failure. If the fault returns promptly, move on to the steps below.
- Check that the gas supply to the property is working DIY safe
Try a gas hob or another gas appliance. If they are also not working, the issue is with the gas supply rather than the boiler — contact your gas supplier. If other gas appliances work normally, the gas supply is fine.
- Visually inspect the boiler for obvious wiring issues DIY safe
With the boiler switched off at the mains, open the boiler casing (where accessible without tools and safe to do so) and look for any wiring that appears burnt, melted, or clearly disconnected. Do not touch or reconnect any components yourself — simply note what you see and report it to the engineer. If you are not comfortable doing this, skip straight to calling an engineer.
- Have a Gas Safe engineer inspect the flow NTC sensor and its connections Gas Safe engineer
An engineer will locate the flow temperature sensor on the primary circuit, check the resistance reading against the expected value (approximately 10 kΩ at 25°C, falling as temperature rises), and inspect both plug connections — at the sensor end and at the PCB. This confirms whether the sensor, the wiring, or the board is at fault.
- Replace the flow NTC thermistor if found to be faulty Gas Safe engineer
If the sensor is reading outside its correct resistance curve or producing no signal at all, the engineer will drain down the relevant section of the boiler, remove the old sensor, and fit a replacement. Because Saunier Duval no longer trades in the UK (the brand is part of the Vaillant Group), the engineer should confirm part availability before starting work — compatible aftermarket sensors are available, though genuine parts may need to be sourced through specialist suppliers.
- Inspect and repair sensor wiring if the sensor tests correctly Gas Safe engineer
If the sensor resistance is within specification, the engineer will trace the wiring loom from the sensor to the PCB, checking for breaks, shorts, or damaged insulation. Faulty wiring is repaired or replaced as needed.
- Test or replace the PCB if sensor and wiring are both sound Gas Safe engineer
Should both the sensor and the wiring check out, the fault points to the control board. PCB diagnosis and replacement is specialist work — the engineer will advise on cost versus the age and value of the boiler before proceeding, as a PCB replacement on an older Saunier Duval can sometimes make a boiler replacement more cost-effective.
Parts you may need
- Flow NTC thermistor (temperature sensor) · from £18
- Sensor wiring loom / harness · from £35
- PCB (control board) · from £180
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–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F.00 fault myself?
The safe homeowner checks are limited to resetting the boiler once or twice and confirming your gas supply is working. The actual diagnosis and repair — checking sensor resistance, reseating connectors, replacing the NTC thermistor, or repairing wiring — must be carried out by a Gas Safe registered engineer. These tasks involve accessing internal boiler components and electrical connections, and attempting them without the right qualifications is unsafe and will likely void any remaining warranty.
How much does it cost to fix a Saunier Duval F.00 fault in the UK?
For a straightforward flow NTC sensor replacement, most UK homeowners pay in the region of £100–£280, covering the engineer's call-out, labour, and the part. The sensor itself is an inexpensive component, so it is largely the labour and call-out fee that drives the bill. Costs are typically 20–35% higher in London and the South East. Out-of-hours or emergency call-outs attract a further premium of 25–50%. In the rare situation where the PCB needs replacing, costs can rise considerably — this is worth discussing with your engineer before authorising the work on an older boiler.
Saunier Duval no longer seem to trade in the UK — can I still get parts?
Saunier Duval is part of the Vaillant Group and the brand is no longer actively sold in the UK, which means genuine parts can be harder to source than they would be for a current manufacturer. However, compatible aftermarket NTC sensors are widely available from UK boiler parts suppliers and are typically a reliable and affordable option. Ask your engineer to confirm part availability before they attend, to avoid delays.
Could the F.00 fault damage my boiler if I leave it?
The boiler locks out rather than continuing to run without a reliable temperature reading, which is actually a safety feature — it prevents the system overheating. However, leaving the fault unresolved for an extended period means no heating or hot water. It is also worth noting that if the underlying cause is a wiring fault rather than a sensor, continued damp or vibration could worsen the damage over time. Book an engineer promptly, especially in colder months.