Saunier Duval F.79 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.79 fault code mean?
The F.79 fault code on Saunier Duval boilers indicates that the control board cannot obtain a valid temperature reading from the domestic hot water (DHW) cylinder or accumulator temperature sensor. Without this signal, the boiler cannot safely regulate hot water production, so it shuts down as a precaution. The root cause is almost always an electrical issue — a loose or corroded sensor connector, a break somewhere in the wiring harness, or a sensor (NTC thermistor) that has failed outright. In rare cases, a faulty PCB detection circuit or incorrect product coding can produce the same code.
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
- Loose or corroded sensor connector Common
The plug connecting the DHW cylinder NTC sensor to the wiring harness can work loose through vibration over time, or the pins can corrode — particularly in damp plant rooms. Either way, the control board receives no usable signal and triggers F.79.
- Broken or damaged wiring harness Common
The cable run between the sensor and the PCB can suffer chafing, pinching, or a snapped conductor, especially where wiring passes through tight channels or near components that generate heat. Even a partial break in one wire is enough to cause an intermittent or permanent open circuit.
- Failed NTC thermistor (temperature sensor) Sometimes
NTC thermistors degrade electrically over years of use. When a sensor fails open-circuit or short-circuit, the resistance reading falls completely outside the expected range and the boiler locks out. Because replacement sensors are inexpensive, fitting a new one is almost always the preferred fix.
- Incorrect product or sensor coding Rare
On certain Saunier Duval models, a mismatch between the product coding resistor or the user-interface board and the main PCB can cause the control board to misinterpret — or fail to detect — the sensor input, producing an F.79 lockout even when the sensor itself is sound.
- Faulty PCB sensor detection circuit Rare
If the wiring and the sensor both test correctly with a multimeter but F.79 persists, the input circuit on the PCB that reads the sensor signal may itself have failed. This is the least likely cause but should be investigated once all other possibilities have been ruled out.
How to fix it
- Check that the boiler's gas and electrical supplies are on DIY safe
Confirm the gas isolation valve on the supply pipe to the boiler is fully open and that no other gas appliances in the property have lost supply. Also check that the boiler has not tripped a circuit breaker. These are simple checks that rule out an unrelated supply interruption before you proceed.
- Reset the boiler (once or twice only) DIY safe
Locate the reset button — usually marked with a flame and crossed-out symbol or labelled 'Reset' — and hold it for the duration described in your user manual (typically 3–5 seconds). If the boiler fires up and runs normally, monitor it closely. If F.79 returns within a short time, do not keep resetting: repeated lockouts indicate a genuine fault that needs an engineer.
- A Gas Safe engineer inspects the DHW sensor connector and wiring harness Gas Safe engineer
The engineer will open the boiler casing and trace the wiring from the DHW cylinder NTC sensor back to the PCB, checking each connector for loose pins, corrosion, or damaged insulation. This step alone resolves F.79 in a significant proportion of cases and is inexpensive if the only remedy is re-seating a plug or repairing a chafed wire.
- Engineer tests the NTC thermistor resistance Gas Safe engineer
Using a multimeter set to measure resistance, the engineer checks the sensor's ohm reading at a known ambient temperature and compares it to the manufacturer's specification chart. An open-circuit reading or a value far outside the expected range confirms sensor failure, while a reading within spec points the investigation back to wiring or the PCB.
- Engineer replaces the faulty NTC sensor if required Gas Safe engineer
If the thermistor has failed, the engineer will fit a compatible replacement sensor. NTC thermistors for Saunier Duval systems are relatively low-cost parts, so replacement is almost always more practical than attempting any kind of repair to the sensor itself.
- Engineer checks product coding and board compatibility Gas Safe engineer
Where the wiring and sensor both pass testing, the engineer will verify that the coding resistor and any fitted user-interface board match the main PCB settings for the specific boiler model. An incorrect coding configuration can be corrected by the engineer without major parts expenditure.
- PCB replacement if all other components test correctly Gas Safe engineer
Should the sensor, wiring, and coding all prove satisfactory but F.79 continues, the sensor-input circuit on the main PCB is the likely culprit. PCB replacement is the most expensive possible outcome; discuss the repair cost versus boiler age and condition with your engineer before committing.
- Call a Gas Safe registered engineer if you have not already done so DIY safe
All internal inspection, testing, and component replacement on a gas boiler must be carried out by a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. Do not attempt to open the boiler casing or handle any internal wiring yourself.
Parts you may need
- DHW cylinder NTC temperature sensor (thermistor) · from £35
- Sensor wiring harness / loom · from £55
- Sensor connector plug and pins · from £12
- Main PCB (control board) · from £220
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 £110–£300, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F.79 fault code myself?
The only DIY actions that are safe are checking that your gas supply is on and attempting a reset of the boiler once or twice. All internal work — inspecting connectors, testing the sensor with a multimeter, or replacing any component — must be done by a Gas Safe registered engineer. It is a legal requirement in the UK for internal gas appliance repairs to be carried out by a registered professional.
How much does it typically cost to fix an F.79 fault on a Saunier Duval boiler?
For the most common repairs — re-seating a loose connector or replacing the DHW NTC sensor — you should expect to pay in the region of £110 to £300 including parts and labour. A straightforward wiring repair sits towards the lower end of that range, while a sensor swap with a standard callout sits comfortably in the middle. If the PCB itself turns out to be at fault, costs rise considerably — typically £360 to £790 — so it is worth discussing with your engineer whether replacement makes more financial sense than repair on an older boiler.
Will resetting the boiler clear the F.79 code permanently?
A reset may clear the lockout temporarily, but if the underlying fault — a failed sensor, a broken wire, or a loose connector — has not been fixed, F.79 will return. Avoid resetting more than two or three times in quick succession. Repeated forced restarts without addressing the cause can place unnecessary stress on other components and turn a modest repair bill into a larger one.
My Saunier Duval boiler still has hot water — why has it locked out with F.79?
The F.79 code relates specifically to the temperature sensor on the DHW cylinder or accumulator, not to the boiler's primary heat exchanger. On some system configurations the boiler may lock out DHW production as a precaution even if residual heat in the cylinder provides short-term hot water. The lockout is a safety measure: without a valid sensor reading, the control board cannot regulate cylinder temperature reliably, which could in principle lead to scalding-hot water or inefficient operation.