Saunier Duval F13 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F13 fault code mean?
The F13 fault code on a Saunier Duval boiler points to a problem with the main control board (PCB). The boiler's internal diagnostics have detected either a failure of the board itself, a fault in its wiring connections, or a component that communicates with it — such as the domestic hot water storage tank sensor. When F13 appears the boiler will typically lock out completely and refuse to fire until the underlying issue is resolved. Because Saunier Duval and Glow-Worm share a platform within the Vaillant Group, the same code is sometimes described on shared models as a 'PCB main connections fault', highlighting that wiring and connector integrity is just as important to check as the board itself. Older Saunier Duval models may also show F13 when the relay that switches between central heating and domestic hot water modes has failed, so the exact cause can vary by model age and specification.
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 wiring connections to the PCB Common
Vibration over time, or disturbed wiring during a previous service or accessory installation, can leave connectors partially unseated or terminal pins corroded. The board detects a break in its expected signal paths and triggers F13. This is often the first thing an engineer will check because it is straightforward to inspect and correct without replacing any components.
- Failed main PCB / control board Common
The PCB can fail outright due to a power surge, component aging, or moisture ingress. Visible signs may include burn marks or discolouration on the board, but internal micro-circuit cracks can also occur — these open and close as the board heats and cools, causing F13 to appear intermittently or only when the boiler has been running for a while. A failed PCB will not respond to resets and requires professional replacement.
- Faulty DHW storage tank sensor Sometimes
On Saunier Duval system boilers connected to a separate hot water cylinder, the tank sensor sends temperature data back to the PCB. If this sensor develops a fault or its wiring becomes intermittent, the PCB can interpret the missing or erratic signal as a board error and display F13. The fault may only appear when the boiler enters domestic hot water mode or even just sits on standby, which is a useful clue for the diagnosing engineer.
- Failed heating/DHW changeover relay (older models) Sometimes
On older Saunier Duval boilers the relay responsible for switching the boiler's output between the central heating circuit and the domestic hot water circuit can wear out or stick. When this relay fails the PCB cannot correctly manage the two demands and logs F13. This is more characteristic of equipment that is ten or more years old.
- Transient software glitch or power interruption Rare
A brief mains power fluctuation or a voltage spike can cause the PCB's firmware to enter an error state and display F13 without any underlying hardware fault. In this scenario a full power-down reset will clear the code permanently. If the code returns after resetting, a hardware cause should be assumed.
How to fix it
- Reset the boiler using the reset button or by switching it off at the mains for 10–15 minutes DIY safe
A sustained power-down (rather than a quick switch-off) allows the PCB's capacitors to fully discharge and the firmware to reinitialise. After restoring power, observe whether the boiler fires normally. If F13 does not return after one or two resets, a transient electrical glitch may have been the cause. Do not reset more than two or three times — repeated resets without a successful start indicate a genuine fault that needs investigation.
- Check that the boiler's mains electricity supply is stable and that no other sockets or circuits in the home are showing signs of power issues DIY safe
A power surge or unstable voltage can damage the PCB. If you have experienced a power cut or electrical storm recently, note this when speaking to the engineer as it is diagnostically useful. If voltage instability is a recurring issue in your property, an engineer may recommend a surge-protection device on the boiler's supply.
- Do not attempt to open the boiler casing, inspect the PCB, or disturb any internal wiring Gas Safe engineer
Checking and reseating PCB connectors, inspecting for burn marks, and testing wiring harnesses with a multimeter are diagnostic steps that must be carried out by a Gas Safe registered engineer. Interfering with internal wiring without the correct training can create unsafe conditions and will invalidate any remaining warranty.
- Contact a Gas Safe registered engineer to carry out a full electrical diagnosis Gas Safe engineer
The engineer will power down the boiler safely, inspect all wiring looms and connector blocks for loose pins, corrosion, or damage, and test the PCB's input and output voltages with a multimeter. On models with a DHW storage tank they will also check the tank sensor's resistance reading. This systematic approach identifies whether the fault lies in wiring, a sensor, the relay, or the board itself — avoiding unnecessary parts replacement.
- Have the engineer replace the faulty component — PCB, sensor, relay, or wiring harness as identified Gas Safe engineer
If the PCB itself has failed the engineer will source a compatible replacement board for your specific Saunier Duval model, fit it, and carry out a full commissioning check. If a DHW sensor or changeover relay is at fault, these are typically less costly to replace than the whole board. Once the repair is complete the engineer should reset and test all boiler functions including central heating and hot water modes.
Parts you may need
- Main PCB / control board (Saunier Duval compatible) · from £250
- DHW storage tank sensor · from £35
- Heating/DHW changeover relay · from £45
- Wiring harness / connector loom · from £60
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–£450, depending on the underlying cause.
Frequently asked questions
Can I fix an F13 fault myself on a Saunier Duval boiler?
The only safe DIY step is to attempt a full power-down reset. Everything beyond that — opening the casing, checking wiring, testing the PCB, or replacing any component — must be carried out by a Gas Safe registered engineer. Working on the internal electrics of a gas boiler without the correct qualifications is unsafe and illegal in the UK.
My F13 fault only appears when the boiler is in hot water mode — why?
This is a recognised pattern on Saunier Duval system boilers connected to a hot water cylinder. The most likely causes are a failing DHW tank sensor sending an erratic signal, or a worn changeover relay that struggles when it switches from heating to hot water mode. Tell your engineer exactly when the fault appears — it narrows the diagnosis significantly and can save time and money.
How much does it cost to fix a Saunier Duval F13 fault in the UK?
Most people pay between £120 and £450. A wiring or connector repair at the lower end can cost around £120–£200 including call-out. A sensor or relay swap typically falls in the £150–£280 range. A full PCB replacement — parts and labour — generally comes to £300–£500, though on some older or less common models where the board is hard to source, costs can be higher. Engineers in London and the South East tend to charge towards the top of these ranges.
Is it worth repairing an F13 fault or should I replace the boiler?
If your boiler is under ten years old and otherwise in good condition, repair is almost always worthwhile — especially if the fault turns out to be a sensor or wiring issue rather than the PCB itself. If the boiler is over twelve years old and a full PCB replacement is needed, it is worth getting a new boiler quote at the same time. As a rule of thumb, if the repair cost exceeds half the price of a comparable new boiler installation, replacement becomes the more sensible long-term investment.