Vaillant F.73 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.73 fault code mean?
The F.73 fault code indicates that the boiler's printed circuit board (PCB) is receiving no usable signal from the water pressure sensor — the reading is either absent or so low it falls outside the accepted range. This typically points to a break or short-circuit in the wiring between the sensor and the PCB, or to the sensor itself having failed. Because the pressure sensor plays a critical role in confirming there is sufficient water pressure before the boiler can fire safely, the boiler immediately shuts down and enters lockout mode when this signal is lost. In most cases the boiler will refuse to restart via a reset until the underlying fault is resolved, though intermittent wiring problems can occasionally allow the boiler to run sporadically before the fault reappears.
General guidance only — not a substitute for professional advice. 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
- Leaking pressure sensor corroding the wiring connector Common
This is the most frequent root cause seen in practice. The pressure sensor develops a small weep or leak over time, which goes unnoticed inside the boiler casing. The escaping water gradually corrodes the connector on the wiring harness where it plugs into the sensor, breaking the electrical connection and triggering F.73. Engineers will often spot a tell-tale green or white oxidation on the connector pins.
- Low system water pressure Common
If the system pressure drops below approximately 0.6 bar, the pressure sensor output falls outside the range the PCB expects and the boiler raises F.73 as a safety response. Check the boiler's pressure gauge — a healthy system should read between 1 and 1.5 bar when cold. A reading noticeably below 1 bar suggests the system needs repressurising via the filling loop.
- Damaged, loose, or water-damaged wiring Sometimes
The wiring harness connecting the pressure sensor to the PCB can suffer from chafing, loose plug connections, or water ingress from an unrelated internal leak. Any of these can interrupt the signal and produce F.73. The fault may appear intermittently if a connector is only partially disrupted.
- Faulty or failed pressure sensor Sometimes
The sensor itself can fail electronically without any visible leaking. The internal components degrade over time, particularly on older boilers, and eventually the sensor produces no valid output. Replacing the sensor unit resolves the fault in these cases.
- Internal boiler leak from pump seals or heat exchanger Sometimes
A leaking pump seal or cracked heat exchanger can allow water to track along cables and into electrical connectors throughout the boiler. This water damage may affect the pressure sensor circuit among others, causing F.73 alongside other symptoms such as visible damp inside the casing.
- Faulty PCB Rare
In a small number of cases the PCB itself is unable to correctly read the pressure sensor signal even when the sensor and wiring are intact. This is generally diagnosed by a process of elimination after the sensor and wiring have been verified or replaced.
How to fix it
- Check the system pressure gauge DIY safe
Locate the pressure gauge on the front of the boiler — it is usually a small dial or a digital readout. The needle or reading should sit between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, low pressure may be contributing to or directly causing the F.73 fault.
- Repressurise the system via the filling loop if pressure is low DIY safe
If the pressure is below 1 bar, use the boiler's filling loop (a braided flexible hose or a keyway valve beneath the boiler) to slowly add water until the gauge reads approximately 1.2–1.5 bar. Close the filling loop valves once the correct pressure is reached. Consult your boiler's user guide for the exact procedure for your model.
- Attempt a boiler reset DIY safe
Once pressure is at the correct level, press and hold the reset button (or follow the reset procedure in your user manual) for around three seconds. The boiler should attempt to restart. If F.73 clears and the boiler fires up normally, monitor the pressure over the following days to check it is holding. Do not attempt more than two or three resets — repeated resetting without a successful restart will not resolve an underlying component fault.
- Check for visible signs of a water leak inside or around the boiler DIY safe
With the boiler powered off, carefully inspect the area beneath and around the boiler casing for damp patches, water stains, or drips. If you can safely remove the boiler's front panel (some Vaillant models allow this without tools), look for signs of moisture near the pressure sensor and its wiring connector. Do not touch any wiring or components — simply note what you see to report to an engineer.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the fault persists after repressurising and resetting, or if pressure was already correct when F.73 appeared, the repair requires a qualified engineer. They will use diagnostic tools to test the pressure sensor output, inspect the wiring harness and connectors for corrosion or damage, and determine whether the sensor, wiring, or PCB needs to be replaced. Attempting to replace electrical components inside the boiler yourself is unsafe and may invalidate your warranty.
Parts you may need
- Vaillant water pressure sensor · from £35
- Wiring harness / loom (model-specific) · from £55
- Filling loop assembly · from £20
- PCB (printed circuit board) · from £180
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–£350, depending on the underlying cause.
Frequently asked questions
Can I reset a Vaillant F.73 fault myself?
You can try a reset, but in most cases F.73 will return immediately or the boiler will refuse to restart altogether. The only DIY step worth trying first is checking and topping up the system pressure to around 1.2–1.5 bar via the filling loop, then attempting a reset. If that does not clear the fault, the underlying cause — a faulty sensor, corroded connector, or damaged wiring — needs a Gas Safe engineer to fix properly.
How much does it cost to fix a Vaillant F.73 fault in the UK?
Most homeowners pay somewhere between £120 and £350 all-in, covering the engineer's call-out, labour, and parts. A pressure sensor replacement with wiring connector clean-up sits at the lower end of that range, while a full wiring harness replacement pushes toward the higher end. A PCB replacement is considerably more expensive and is only needed in a small minority of cases — if an engineer suggests this, it is worth getting a second opinion or weighing up the boiler's age before proceeding.
Why does my Vaillant keep showing F.73 even after I top up the pressure?
If the system pressure was already in the normal range, or if topping up did not clear the fault, the issue is almost certainly the pressure sensor itself or its wiring connection rather than genuinely low pressure. The most common scenario is a slow leak from the sensor that has corroded the wiring connector, meaning the PCB simply cannot receive a valid signal regardless of the actual pressure in the system. An engineer will need to inspect the sensor, clean or replace the connector, and test the circuit.
Is a Vaillant boiler showing F.73 worth repairing or should I replace it?
In the vast majority of cases F.73 is absolutely worth repairing — a pressure sensor or wiring fix is a relatively modest cost compared to a new boiler installation. The exception is if the boiler is already over 10–12 years old and the engineer finds that the fault has been caused by a more serious internal leak from the heat exchanger or pump, or that the PCB also needs replacing. In those circumstances it may be more economical to invest in a new, more efficient appliance. Ask your engineer for an honest assessment based on the boiler's overall condition.