Protherm F.73 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.73 fault code mean?
The F.73 fault code on a Protherm boiler indicates that the central heating water pressure sensor is either not communicating correctly with the control board, or its signal has fallen outside the accepted range. In practical terms, the boiler's electronics are seeing a reading that suggests the sensor cable has broken, developed a short-circuit to 0V, or the sensor itself has failed completely. Because the boiler cannot confirm safe water pressure, it shuts down as a precautionary measure. Protherm is part of the Vaillant Group, and this fault code carries the same meaning across both brands — so if you own a Vaillant and see F.73, the diagnosis and remedy are essentially identical.
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
- Corroded wiring connector at the pressure sensor Common
This is the single most frequent cause of F.73 in practice. If the pressure sensor has been weeping water — even a very small, slow drip — the moisture works its way along the cable and into the electrical connector, causing corrosion that disrupts the signal. The corrosion can sometimes be cleaned and the connector salvaged, saving the cost of a full wiring harness replacement.
- Failed or worn water pressure sensor Common
Like any electromechanical component, the pressure sensor degrades over time. Internal membrane failure or general wear can cause it to output a signal that the PCB rejects as out of range, triggering F.73 even when the system pressure is perfectly normal. Sensors are relatively inexpensive parts and straightforward for an engineer to swap out.
- Loose or displaced wiring connection Sometimes
Vibration during normal boiler operation, combined with repeated thermal expansion and contraction of components, can gradually work a connector loose. A partially seated plug will produce an intermittent or absent signal that the PCB interprets as a sensor fault.
- Internal water leak damaging the wiring harness Sometimes
A leak elsewhere inside the boiler — from the heat exchanger, pump seals, or pipe joints — can allow water to track along the wiring loom and reach the sensor connector or even the PCB. This type of fault usually requires the engineer to fix the leak source first before addressing the electrical damage.
- Faulty PCB Rare
If the pressure sensor and all wiring check out correctly, the control board itself may have developed a fault on the circuit that reads the sensor input. PCB failure is a less common cause of F.73, but should be considered once all other components have been ruled out by an engineer.
How to fix it
- Check the boiler display and note the exact code DIY safe
Confirm the display reads F.73 (sometimes shown as F73). Make a note of how frequently the fault appears — a code that clears on reset but returns within hours points more strongly to a genuine sensor or wiring issue than a one-off occurrence.
- Reset the boiler once or twice DIY safe
Locate the reset button (usually marked with a flame symbol or labelled 'Reset') and hold it for a few seconds until the boiler attempts to restart. If the F.73 clears and the boiler runs normally, monitor it over the next 24 hours. A fault that returns promptly after reset is unlikely to resolve itself and will need an engineer.
- Check and top up system pressure if it is below 1 bar DIY safe
Look at the pressure gauge on the boiler — it should sit between 1 and 1.5 bar when the system is cold. If it has dropped below 1 bar, use the filling loop (the braided silver hose under or near the boiler) to top it up slowly until the gauge reads around 1.2 bar, then close the loop taps. Note that F.73 is a sensor signal fault rather than a low-pressure alert, so re-pressurising alone is unlikely to clear it — but ruling out a pressure problem is a sensible first step.
- Visually inspect around the pressure sensor for signs of moisture or dripping DIY safe
Without touching any components, look inside the boiler casing (it is safe to remove the outer casing panels to look, but do not disconnect anything). If you can see corrosion staining, limescale deposits, or damp patches near the sensor or its wiring, this strongly suggests the connector corrosion cause and is useful information to pass on to your engineer.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
All hands-on work from this point — unplugging and testing the sensor, checking wiring continuity, cleaning or replacing the connector, fitting a new sensor, or inspecting the PCB — must be carried out by a Gas Safe registered engineer. They will use a multimeter to confirm whether the sensor is producing the correct resistance or voltage output, trace any wiring damage, and replace whichever component has failed. You can verify an engineer's credentials at GasSafeRegister.co.uk.
Parts you may need
- CH water pressure sensor · from £22
- Sensor wiring harness / connector loom · from £45
- Boiler 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 £100–£220, depending on the underlying cause.
Frequently asked questions
Can I fix Protherm F.73 myself by just resetting the boiler?
A reset is the only step a homeowner should attempt, and it is worth trying once or twice. If the fault clears and the boiler stays running, keep an eye on it. However, if F.73 returns within a short time, resetting repeatedly will not resolve the underlying problem — you will need a Gas Safe registered engineer to inspect the sensor and wiring.
How much will it cost to fix Protherm F.73 in the UK?
Most F.73 repairs fall in the £100–£220 range. If the fix is simply cleaning a corroded connector, the bill will be at the lower end. Replacing the pressure sensor itself adds a modest parts cost (around £15–£30 for the sensor) on top of labour. If the fault turns out to be a PCB failure — which is less common — the cost can rise significantly, potentially £300–£450 or more depending on the board, but this is not the typical outcome for F.73.
Does Protherm F.73 mean the same as Vaillant F.73?
Yes. Protherm is owned by the Vaillant Group and shares the same underlying boiler architecture and fault code definitions. On both brands, F.73 points to a water pressure sensor signal that is out of range — due to a broken or short-circuited cable, or a defective sensor. The diagnostic and repair approach is the same for both.
Is my boiler worth repairing when F.73 appears, or should I replace it?
For most boilers, F.73 is a relatively straightforward electrical fault involving a sensor or wiring, not a catastrophic failure of a major component like the heat exchanger. Unless your boiler is very old (typically over 15 years), has a history of repeated breakdowns, or the engineer finds additional serious faults during the visit, repair is usually the more cost-effective choice. A Gas Safe engineer can give you an honest assessment once they have diagnosed the root cause.