Saunier Duval F.73 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.73 fault code mean?
The F.73 code appears when the boiler's water pressure sensor sends a signal that falls outside the expected range, or when the sensor connection to the control board is interrupted or short-circuited. In plain terms, the boiler can no longer trust the reading it's getting from the pressure sensor — so it shuts down as a precaution. You'll typically see the F.73 code flashing on the display, with no heating or hot water available, and the fault may return immediately after a reset if the underlying cause hasn't been addressed. Saunier Duval boilers are part of the Vaillant Group, and this fault code carries the same meaning across compatible models in both ranges.
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 water pressure sensor Common
The pressure sensor itself is the most common culprit. Over time, sensors degrade due to age, scale build-up, or internal wear, causing them to produce inaccurate or out-of-range readings. When the control board receives a signal it can't make sense of, it triggers F.73 and locks out. The standard replacement part is referenced under Saunier Duval part number S5720500.
- Loose, corroded, or moisture-damaged wiring connections Common
The plug connecting the pressure sensor to the PCB can work loose due to vibration and thermal cycling, or become corroded over time. In many cases, moisture from a nearby internal leak finds its way into the connector, causing signal interference. Encouragingly, over 60% of F.73 call-outs are resolved simply by reseating the connector or drying out the plug — no new parts required.
- Internal water leak damaging the wiring harness Sometimes
A slow leak from the heat exchanger, pump seals, or a corroded pipe joint can allow water to track along the wiring loom that connects the sensor to the PCB. This causes corrosion on the wire terminals and disrupts the sensor signal, triggering F.73. A visible damp patch or mineral staining inside the boiler casing is a tell-tale sign.
- Faulty PCB (printed circuit board) Rare
If the pressure sensor and its wiring test as healthy, the PCB may be failing to correctly interpret the sensor signal. This is a less common cause of F.73 but should be considered once all other components have been checked and cleared.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button as described in your boiler's user guide. If the F.73 code clears and the boiler fires up normally, monitor it over the next few hours. If the fault returns promptly after reset, move on to the next checks rather than repeatedly resetting — more than two or three resets without success won't help and could mask the problem.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler (or your heating system). For a Saunier Duval boiler at rest (cold), the pressure should sit between 1.0 and 1.5 bar. If it reads below 0.8 bar, the system needs topping up. Locate the filling loop — usually a silver braided hose with a small valve beneath the boiler — and slowly open the valve until the gauge rises to around 1.2–1.5 bar, then close it firmly. Do not exceed 2 bar.
- Bleed the radiators if pressure drops repeatedly DIY safe
If you find yourself needing to top up pressure regularly, trapped air may be escaping and pulling pressure down. Turn the heating off and allow it to cool. Using a radiator bleed key or flat-head screwdriver, open each radiator's bleed valve (starting with the lowest floor) just enough to let air hiss out, then close it once water appears. Re-check and top up system pressure afterwards if needed.
- Look for signs of a water leak DIY safe
With the boiler casing on (do not remove it), check the area around and beneath the boiler for drips, damp patches, or white limescale staining on pipework. Also check nearby radiator valves and any visible pipework. If you spot a leak, do not attempt to fix it yourself — note its location and report it to a Gas Safe engineer.
- Gas Safe engineer: inspect and reseat the sensor wiring Gas Safe engineer
With the boiler isolated from power, the engineer will trace the cable running from the pressure sensor to the PCB, checking for chafing, pinching, burnt insulation, or a damaged sheath. A multimeter in resistance mode will confirm whether continuity is intact. If the connector has worked loose or moisture is present, reseating or cleaning the plug often clears the fault without any parts being replaced.
- Gas Safe engineer: test and replace the pressure sensor if required Gas Safe engineer
The engineer will disconnect the sensor connector and measure the resistance across its pins, comparing the result against the specification for your specific model. A zero-ohm reading indicates an internal short; an infinite reading indicates an open circuit. In either case — or if scale or debris has blocked the sensor port — the component needs replacing. The Saunier Duval pressure sensor (part no. S5720500) typically costs £15–£30 in parts, with total labour and parts usually coming in at £80–£200.
- Gas Safe engineer: replace the PCB if all else checks out Gas Safe engineer
If the sensor and wiring are confirmed to be functioning correctly but F.73 persists, the PCB is the remaining suspect. PCB replacement is a last-resort diagnosis and carries a higher cost — typically £180–£400 depending on the model. An experienced engineer will want to be confident the board is genuinely at fault before committing to this repair, particularly on an older boiler where the cost-versus-replacement calculation matters.
- Call a Gas Safe registered engineer if you haven't already Gas Safe engineer
If the homeowner checks above haven't resolved the fault, or if you've found a leak or suspect an electrical issue, contact a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk. Attempting to work on the internal components of a gas boiler without Gas Safe registration is illegal in the UK and potentially dangerous.
Parts you may need
- Water pressure sensor (part no. S5720500) · from £22
- Wiring harness / loom · from £45
- PCB (printed circuit 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 £80–£250, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F.73 fault myself?
Only to a limited extent. As a homeowner you can safely reset the boiler, check and top up the system pressure via the filling loop, bleed the radiators, and look for obvious leaks. However, inspecting or replacing the pressure sensor, checking wiring, or touching any internal electrical components must be carried out by a Gas Safe registered engineer. Working on a gas appliance without registration is illegal in the UK.
How much does it cost to fix a Saunier Duval F.73 fault?
Most F.73 repairs fall in the £80–£200 range, covering a call-out, sensor inspection, and — if needed — a replacement pressure sensor. Interestingly, more than 60% of cases are resolved simply by reseating a loose connector or drying out moisture, so you may not need a new part at all. If the PCB turns out to be the root cause, costs typically rise to £180–£400 depending on the model. On an older boiler, it's worth getting a boiler health check at the same time to weigh up repair versus replacement.
Why does my boiler keep showing F.73 even after I reset it?
If F.73 returns immediately or within a short time after a reset, a simple power-cycle is unlikely to be the solution — the underlying fault is still present. This pattern usually points to a failed pressure sensor sending a consistently out-of-range signal, a persistently loose or corroded connector, or moisture in the wiring. Repeated resets won't cause harm but won't fix it either; you need a Gas Safe engineer to diagnose the root cause.
Is the Saunier Duval F.73 the same fault as Vaillant F.73?
Yes. Saunier Duval is part of the Vaillant Group, and the two brands share the same fault-code definitions across their condensing boiler ranges. F.73 on a Vaillant boiler means exactly the same thing — a water pressure sensor signal that is out of range, short-circuited, or disconnected — and the diagnostic and repair process is essentially identical.