Vaillant F73 is a water pressure sensor fault that shuts the boiler down as a safety measure. It can be caused by low pressure, a faulty sensor, bad wiring or a hidden leak. A handful of safe checks can tell you which of these you are dealing with, but anything that involves opening the boiler casing or working on the sensor itself is a job for a Gas Safe registered engineer.
| What it means | What to do first | Who is needed |
|---|---|---|
| The water pressure sensor signal is too weak, so the boiler has locked out | Check the gauge with the system cold and top up to the normal range if it is low | A Gas Safe registered engineer if F73 returns |
| It can be real low pressure or a sensor, wiring or connection fault | Reset once and look for any sign of a leak | Testing of the sensor, cable, plug and earth |
| Normal pressure with F73 still showing points away from a simple top-up | Do not open the casing or keep resetting | Diagnosis before any part is replaced |
What Does F73 Mean on a Vaillant Boiler?
Vaillant boilers use an electronic water pressure sensor to keep track of the pressure in the sealed heating system. Vaillant’s installation manual describes F73 as the water pressure sensor signal being too weak. In other words, the sensor is reporting a pressure lower than the boiler can safely run on, either because the pressure really has dropped or because a fault in the sensor or its wiring is pulling the signal down. Either way, the boiler locks out instead of carrying on with a reading it cannot rely on.
F73 appears on Vaillant’s ecoTEC boilers and on other Vaillant models that share the same control electronics. On most displays it shows as F.73, with a dot. The exact fault code table can vary by model and software version, so check the list in your own boiler’s user guide to confirm F73 for your specific unit.
F22, F73 or F74? Telling the Pressure Codes Apart
Vaillant uses three codes that all involve water pressure, and it helps to know which one you are looking at.
| Code | What it means | Usual cause |
|---|---|---|
| F22 | Low water pressure | Not enough water in the system, often from a leak or after bleeding radiators |
| F73 | Pressure sensor signal too weak | Low pressure, or a short in the sensor, its cable, its plug or its earth connection |
| F74 | Pressure sensor signal too strong | Pressure too high, often from overfilling, or a sensor or wiring fault |
If the boiler fires but the sensors see little or no rise in temperature, you may see a different code instead. Our guide to the Vaillant F83 fault code covers that one, which is also often linked to low water in the system.
Is It Low Pressure or a Faulty Sensor?
This is the question that decides what happens next, and the pressure gauge on the front of the boiler is the quickest way to answer it.
| Situation | What it suggests |
|---|---|
| Gauge reads low | Low pressure, so top up and watch it |
| Gauge normal but F73 stays | Sensor, wiring or board |
| Pressure drops again after a top-up | A leak somewhere in the system |
If the gauge sits where it should and F73 still will not clear, adding more pressure will not fix it, and repeated resets only delay a proper diagnosis. If the pressure keeps falling after you have topped it up correctly, the system is losing water somewhere, which points to a leak.
Common Causes of F73
A handful of faults account for most F73 lockouts. Knowing the usual pattern each one follows helps make sense of what an engineer finds once they test the system.
| Cause | Typical sign |
|---|---|
| Low system pressure | The gauge reads below 1 bar with the system cold |
| Faulty pressure sensor | F73 appears even with correct, stable pressure |
| Damaged or wet wiring | The fault is intermittent, clearing and then returning |
| Loose sensor plug or earth connection | The fault appears after a knock, vibration or recent work on the boiler |
| Internal leak, such as a worn pump seal | Pressure drops gradually with no visible leak nearby |
| Corrosion on the sensor or its connection | The boiler is older, or the system has not been treated with inhibitor |
Some of these, like a loose plug, are minor once an engineer has access to the boiler. Others, such as a pump seal that has started to weep, need a proper diagnosis before anyone can say what the repair involves.
What You Can Safely Check Yourself
There is a short list of checks that do not involve opening the boiler casing, and they are worth doing before you book anyone.
- Reset the boiler once, following the instructions in your user manual, and see whether F73 returns straight away.
- Check the pressure gauge with the heating off and the system cold. Vaillant gives the normal cold pressure as 1 to 1.5 bar, but check the exact figure and gauge markings for your model in the handbook.
- If the pressure is low, top up slowly using the filling loop, a little at a time, watching the gauge so you do not overfill. Overfilling can trigger the opposite fault, F74. Once the gauge reaches the right level, close both filling loop valves fully, and if your loop is the removable type, disconnect it. If the pressure keeps dropping however many times you top it up, stop and call a Gas Safe engineer.
- Look around the boiler, radiators and visible pipework for any sign of a leak, such as damp patches, drips or limescale staining.
These checks stop at the filling loop and the outside of the boiler. Do not open the boiler casing, do not try to test or bypass the pressure sensor yourself, and do not keep resetting the boiler if F73 keeps coming back. Repeated resets do not fix a sensor, wiring or leak fault, and they can delay you noticing a problem that is getting worse.
When to Call a Gas Safe Engineer
Book an engineer if the fault returns after a reset, if the pressure reads correctly but F73 still appears, if you find any sign of a leak, or if the checks above do not clear it. Any of these point to something that needs testing with the boiler open.
An engineer attending an F73 fault will usually check the system pressure, then test the pressure sensor, its cable, its plug and its earth connection, which are the checks Vaillant lists for this code. If the pressure keeps dropping, they will also look for the source of the leak. That combination of tests is what shows whether you need a new sensor, a wiring repair or something else, which the fault code alone cannot tell you. Our boiler repair in London engineers run exactly these checks before any repair begins.
Is Your Vaillant Still Under Guarantee?
If your boiler is still within its Vaillant guarantee, check your paperwork before you book a repair. A failed pressure sensor is a fault with a part of the boiler, so it may be covered. Low pressure or a leak elsewhere in the heating system is not a fault with the boiler itself, and Vaillant may charge for a visit when the problem turns out not to be with the product. That is one more reason to check the pressure gauge and top up first. Vaillant’s longer guarantees also usually depend on the boiler being registered after installation, so confirm the registration if you are not sure.
How Much Does an F73 Repair Cost?
There is no single price for an F73 repair, because the code covers several quite different problems. A pressure top-up that turns out to be the whole issue costs nothing. A faulty sensor or a wiring repair takes more time and may need a replacement part. A leak that has to be traced, particularly inside the boiler, generally takes longer again. An engineer can only give you an accurate figure once they have opened the boiler and found the cause.
BeFix charges £95 an hour between 7am and 5pm, £125 an hour between 5pm and 9pm, and £160 an hour between 9pm and 7am, all plus VAT, with no call-out fee. Any replacement part is costed separately and confirmed after the fault has been diagnosed.
Preventing F73 Coming Back
Get into the habit of glancing at the pressure gauge every few weeks, so you notice a slow drop before it causes a lockout. If you bleed your radiators, check the pressure again afterwards, because bleeding lets water out of the system. An annual boiler service gives an engineer the chance to spot a weeping seal, a corroding connector or a sensor on its way out. A magnetic system filter and the right level of inhibitor in the water also slow down the corrosion that can affect the sensor and its connections over time.
None of this guarantees F73 will never return, but it cuts down on the gradual pressure loss and corroded connections behind most cases. If F73 keeps coming back, BeFix (Gas Safe 967796) repairs boilers across London. Call 020 8058 4241 to book an engineer.
Frequently Asked Questions
What does F73 mean on a Vaillant boiler?
F73 means the water pressure sensor signal is too weak. The boiler has stopped because the sensor is reporting a pressure too low to run on safely. That can be real low pressure, or a fault in the sensor, its wiring or its connection pulling the signal down.
Can I reset a Vaillant F73 fault myself?
You can try resetting the boiler once, topping up the pressure first if the gauge shows it is low. If F73 comes back straight away, or returns within a day or two, stop resetting it and book a Gas Safe engineer, since repeated resets can hide a developing fault.
Is F73 the same as low pressure?
No. Low pressure is one possible cause, but a sensor, wiring or connection fault can trigger the same code with the pressure perfectly normal. If your gauge reads normally and F73 still appears, the fault is most likely with the sensor or its wiring, and topping up or bleeding radiators will not help.
I topped up twice and the pressure looks fine now. Should I still get someone out?
If the pressure has held steady since your last top-up, keeping an eye on the gauge for a week or two is reasonable. If it drops again on its own, the system is losing water somewhere, and that is the point to have the leak traced.
Is it safe to keep using the boiler with F73?
The boiler will not run while F73 is showing, because the lockout is a safety feature. It is fine to do the checks in this guide, but do not keep forcing resets to keep the boiler going, since an unresolved pressure or sensor fault can leave you without heating or hot water again without warning.
Is an F73 fault covered by the Vaillant guarantee?
It can be, if the cause is a faulty pressure sensor or another part of the boiler and the guarantee is still valid. Low pressure or a leak in the wider heating system is not a boiler fault, so a visit for that may be chargeable. Check the pressure first, then check your guarantee paperwork before booking.
How long does an F73 repair take?
If it turns out to be low pressure, topping up and confirming the fix usually takes well under an hour. A faulty sensor or a wiring repair is typically a single visit. If the engineer finds a leak that needs tracing, or a part needs ordering, it can take longer or need a follow-up visit.
How much does an F73 repair cost?
It depends on what is wrong. A pressure top-up costs far less than replacing a sensor, repairing a connection or fixing a leak, and the engineer can only confirm which applies once they have diagnosed the fault in person.
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