Vaillant F73 fault code: water pressure sensor fault explained

What does the Vaillant F73 fault code mean?

The Vaillant F73 fault code means the boiler is receiving a faulty signal from the water pressure sensor, usually because the signal is in the wrong range or the system pressure has dropped too low. It commonly comes down to genuinely low pressure, a leak, a failing pressure sensor, or a wiring or connection problem between the sensor and the boiler's circuit board. You can check the pressure gauge and top the pressure up; replacing the sensor, tracing internal leaks or repairing wiring is an engineer's job.

Vaillant's documentation describes F73 as a fault with the water pressure sensor signal. In plain terms, the boiler relies on that sensor to tell it how much water is in the heating system. When the reading falls outside the expected range, or the sensor cannot communicate properly with the printed circuit board (PCB), the boiler shuts down safely and shows F73 rather than firing up with the wrong information.

Key facts

The quick version, before the detail.

• Meaning: a water pressure sensor signal fault, usually with pressure too low or in the wrong range.
• DIY-fixable? Partly, checking and topping up system pressure and looking for obvious leaks only.
• Gas Safe job if: the sensor, wiring or PCB needs work, or an internal leak needs tracing.
• Most common on: the Vaillant ecoTEC range.
• Smart Plan boiler and central heating cover: parts and labour up to £500 a year if the boiler is under 7 years old, or up to £200 a year if it is over 7.
• A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, if the engineer cannot get access, and for early annual-service requests.
• Cover-plan call-outs run Monday to Friday, 08:00 to 18:00. Outside those hours you are only seen sooner for a genuine emergency breakdown, so a Friday-evening or weekend fault can wait until Monday.
• Using a module starts a 12-month agreement period the first time you use a service. There is a 14-day cooling-off period, but it ends once a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Why does the F73 code appear?

F73 points at the water pressure sensor and the pressure it is reading, rather than the gas side of the boiler. The common causes are genuinely low system pressure, a leak somewhere in the system, a failing or blocked pressure sensor, or loose, corroded or water-damaged wiring between the sensor and the PCB. It shows up most on the Vaillant ecoTEC range.

The table below runs through the likely causes, what you'll notice, and who can safely deal with each one.

F73 causes and who fixes them

Can I fix a Vaillant F73 myself?

Because F73 is about water pressure rather than gas, there are a couple of safe checks you can make before booking anyone. None of them involve opening the boiler or touching the sensor, wiring or PCB, that's the line you never cross. Work through them in this order.

1. Check the pressure gauge on the boiler. If it reads low, usually well below 1 bar, that alone can trigger F73.

2. If pressure is low, top it up using the filling loop, following your Vaillant manual, until the gauge sits at around 1 to 1.5 bar. Do this slowly and don't overfill.

3. Look around the boiler, visible pipework and radiators for obvious leaks or damp patches. If pressure keeps falling after you top up, there is likely a leak.

4. Reset the boiler once, following the steps in your Vaillant manual. Reset only once.

Stop there. Replacing the pressure sensor, repairing wiring, diagnosing the PCB or tracing an internal or hidden leak all mean working inside the boiler or on the heating system, which should be left to a Gas Safe registered engineer. If F73 comes back after you have topped up and reset, book an engineer rather than resetting again.

F73 and F75: how the two pressure codes differ

F73 and F75 are both water pressure faults, so they are easy to mix up. F73 reports that the signal from the water pressure sensor is faulty or in the wrong range. F75 is different: Vaillant defines it as the pressure change being too low when the pump starts up, which usually points at the pump or the sensor not registering the expected rise.

The practical overlap is that both often start with the same homeowner checks, confirm the system has enough pressure and look for leaks, before an engineer decides whether it's the sensor, the pump or the wiring at fault. If you are seeing F75 as well, the underlying issue is likely the same pressure system, so it's worth mentioning both codes when you book.

When to call a Gas Safe registered engineer

If you have topped the pressure up to around 1 to 1.5 bar, checked for obvious leaks and reset once, but F73 keeps coming back, it needs a Gas Safe registered engineer to test the pressure sensor, wiring and PCB and to trace any hidden leak. That's work inside the boiler and the heating system, not a DIY job.

A fault that needs a new pressure sensor, wiring repair or leak tracing is a bigger job than a simple top-up, so the parts and labour can add up. That's worth weighing against how you'd rather cover it, a one-off repair or an ongoing module.

You can book a one-off repair with us and a Gas Safe registered engineer will come and fix it, or call us on 0333 772 6247. A Smart Plan boiler and central heating module covers parts and labour up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it's over 7. If the repair costs more than your cover limit, you would pay anything above the limit yourself. Cover is modular, so you only pick what you want.

A few things worth knowing before you set up cover. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan doesn't cover, if the engineer can't get access, and for early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00; outside those hours you're only seen sooner for a genuine emergency breakdown as set out in your terms, so a fault on a Friday evening or over the weekend can wait until Monday. And using any module starts a 12-month agreement period from the first time you use a service, there's a 14-day cooling-off period, but it ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Smart Plan is a service plan, not insurance. In plain terms, it's a membership that arranges and funds covered repairs up to the limits on your plan, rather than a regulated financial product, it doesn't pay out a cash sum, and cover is limited to the modules you've chosen and the limits set out in your terms. It's provided by UK Boiler Company Ltd, which has traded since 2014 and looked after over 15,000 customers.

One safety note: F73 is a water pressure fault, but if you ever smell gas or suspect carbon monoxide, don't wait for an engineer, call the National Gas Emergency Service on 0800 111 999 straight away.

Vaillant F73 fault code FAQs

Seeing F73? We'll come and fix it for you.

Book a one-off repair and a Gas Safe registered engineer will check the pressure sensor, wiring and any leak, or set up a Smart Plan boiler and central heating module so covered repairs are handled up to your cover limit, up to £500 a year for a boiler under 7 years old, or up to £200 a year if it's older. It's a service plan, not insurance, so it funds covered repairs rather than paying out a cash sum. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan doesn't cover, no-access visits and early annual-service requests. Cover-plan call-outs are Monday to Friday, 08:00 to 18:00 unless it's a genuine emergency. Using a module starts a 12-month agreement period once you first use a service; leaving early costs the remaining months or 75% of the outstanding balance.

Helpful links

• All Vaillant fault codes
• Vaillant F54 gas supply fault
• Vaillant F29 flame-loss fault
• Vaillant F27 fault code
• Boiler not working? Book a repair
• Boiler & central heating cover
• Vaillant official fault-code list
• Check an engineer on the Gas Safe Register