Saunier Duval F.64 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.64 fault code mean?
The F.64 code appears when your Saunier Duval boiler detects a problem with its NTC temperature sensors or the electronics that process their signals. The boiler monitors the temperature of water on both the flow pipe (leaving the heat exchanger) and the return pipe (coming back from the heating circuit). If the readings from these two sensors diverge in a way that doesn't make sense — or if one sensor appears to have short-circuited — the boiler locks out and displays F.64 to protect itself from overheating or running dry. In practice this usually points to a faulty sensor, a wiring connection problem, or a PCB that is misreading the sensor data. Note that Saunier Duval is part of the Vaillant Group alongside Vaillant and Glow-worm, and the F.64 code carries the same meaning across all three brands.
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
- Faulty flow or return NTC thermistor Common
Each NTC sensor is a small temperature-sensitive resistor. When working correctly, a Saunier Duval NTC thermistor reads approximately 3.3 kΩ at room temperature. If the sensor drifts out of range, short-circuits internally, or develops a broken connection at its plug, the PCB receives garbled data and triggers F.64. This is the most frequent cause engineers find on inspection.
- Loose or damaged sensor wiring Common
The thin wires running from the flow and return sensors to the PCB can develop loose push-fit connections, corrosion on the terminals, or chafed insulation — especially where they pass near hot pipework or moving parts. A partial short between wires mimics a sensor fault perfectly and is easy to miss without a methodical wiring inspection.
- PCB fault or water ingress on the board Sometimes
If both sensors test within spec and the wiring is intact, the fault lies with the PCB itself. Moisture from an internal drip — around the pump seal or pressure relief valve — can corrode the board's sensor input circuitry, causing it to misinterpret correct signals. PCB faults are less common than sensor or wiring issues but are more expensive to fix.
- Gas valve or ignition component fault Rare
Experienced engineers note that a cluster of faults in the F.61–F.64 and F.27 range occasionally traces back to the gas valve or, more rarely, the ignition electrode and its connecting cable. This is the least likely cause of F.64 specifically, but a thorough engineer will check these if the sensor, wiring, and PCB all test correctly.
How to fix it
- Reset the boiler once or twice DIY safe
Turn the boiler off at its controls (or at the mains) and leave it for about 30 seconds to allow the electronics to reset fully. Power it back on and attempt a restart using the boiler's reset button. If the F.64 code clears and does not return within a normal heating cycle, monitor the boiler over the next day. If the code comes back — especially quickly — stop resetting and move to the next step. Repeatedly forcing restarts on a locked-out boiler can mask a developing fault.
- Check for any obvious visible issues before calling out DIY safe
While you wait for an engineer, have a look at the boiler casing for any signs of water drips or staining around the pipework, pump, or pressure relief valve outlet. Note whether the fault appeared suddenly or after the boiler had been running for a while, and whether heating, hot water, or both are affected. This information helps the engineer prioritise their diagnosis and can save time on the visit.
- Gas Safe engineer: test flow and return NTC sensors with a multimeter Gas Safe engineer
An engineer will disconnect each sensor from its wiring harness and measure its resistance. A healthy sensor reads around 3.3 kΩ at 20 °C; a short-circuit reading near 0 Ω or an open-circuit reading confirms the sensor has failed. Sensors are relatively inexpensive parts and straightforward to swap on most Saunier Duval models.
- Gas Safe engineer: inspect all sensor wiring and connectors Gas Safe engineer
Even if the sensors themselves test correctly, the engineer should trace the wiring back to the PCB, checking each push-fit connector for corrosion, arcing marks, or terminals that pull free without resistance. A single corroded pin can create an intermittent short that triggers F.64 under load but disappears when the boiler cools.
- Gas Safe engineer: inspect and test the PCB Gas Safe engineer
If sensors and wiring are both in order, the PCB becomes the prime suspect. The engineer will look for visible signs of heat damage, corrosion, or blown components on the board and may swap in a known-good PCB to confirm the diagnosis before ordering a replacement. PCB replacement is the most costly outcome for F.64.
- Gas Safe engineer: check gas valve and ignition components if all else is clear Gas Safe engineer
In the small number of cases where sensors, wiring, and PCB all test satisfactorily, the engineer should verify the gas valve modulation circuit and inspect the ignition electrode and its cable for damage. These components sit within the same diagnostic family as F.64 on Saunier Duval and Vaillant Group boilers.
- Call a Gas Safe registered engineer if the fault persists or returns after a reset DIY safe
F.64 is a lockout that almost always requires professional diagnosis. Use the Gas Safe Register website (gassaferegister.co.uk) to find a qualified engineer local to you. When booking, mention the F.64 code and the boiler model so the engineer can bring likely replacement parts.
Parts you may need
- Flow NTC thermistor sensor · from £18
- Return NTC thermistor sensor · from £18
- PCB (printed circuit board) · from £220
- Sensor wiring harness / loom · from £35
- Gas valve · from £130
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F.64 fault myself?
The only safe DIY step is a reset — turn the boiler off, wait 30 seconds, and restart it once or twice. Everything beyond that (removing the casing, testing sensors with a multimeter, replacing wiring or a PCB) must be done by a Gas Safe registered engineer. Working on a boiler's internal components without Gas Safe registration is illegal and can be dangerous.
How much does it cost to repair an F.64 fault?
Most homeowners pay between £120 and £380 all-in. If the fault is loose wiring or a single NTC sensor, you are typically looking at £120–£180 including the call-out, parts, and labour. A PCB replacement pushes the bill towards £300–£380 for most models. In rare cases where a heat exchanger or extensive electrical work is needed, costs can exceed £500 — if you are quoted that on an older boiler, consider getting a second opinion on whether replacement makes more sense.
Saunier Duval no longer trades in the UK — will I still be able to get parts?
Saunier Duval withdrew from the UK market but their boilers share the Vaillant Group platform, so many components — particularly PCBs, sensors, and pumps — are interchangeable with Vaillant and Glow-worm equivalents. Parts availability is generally reasonable for boilers under about 12 years old. For older models, an experienced heating engineer or specialist supplier can usually identify a compatible Vaillant Group part. If sourcing proves difficult or the repair quote is high, replacement becomes worth considering.
Why does F.64 keep coming back after I reset it?
A code that clears on reset but returns within a heating cycle or two almost always points to a real component fault rather than a one-off electronic glitch. The most likely culprits are an NTC sensor that is borderline — working when cold but drifting out of range as it warms up — or a wiring connection that makes and breaks under thermal expansion. Stop resetting and book an engineer; repeated resets without fixing the underlying cause can occasionally mask a more serious developing fault.