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If your Vaillant boiler has suddenly stopped working and is displaying the F75 fault code, you’re not alone. This is a common issue that often leaves homeowners without heating or hot water. But don’t worry—understanding what causes the F75 error and how to fix it can help you get your boiler back up and running quickly.
In this blog, we’ll cover:
✅ What the Vaillant F75 fault code means
✅ The most common causes of the issue
✅ Ways to diagnose and fix the problem
✅ How much it might cost to repair
By the end of this guide, you’ll know whether you can fix the issue yourself or if you’ll need a Gas Safe engineer to take a look.
The F75 fault code on a Vaillant boiler means that the boiler cannot detect pressure changes in the boiler. In a properly functioning system, the boiler’s pressure sensor should notice a small pressure increase when the pump starts working. If no pressure change is detected, the boiler shuts down as a safety precaution.
🚨 The F75 fault is NOT a DIY fix—this issue requires professional expertise. Unlike simple pressure-related errors, such as topping up the system pressure or bleeding radiators, the F75 fault code involves diagnosing and replacing internal boiler components, which must be handled by a Gas Safe registered engineer for safety and compliance reasons.
The main causes of the F75 error—a faulty pressure sensor or a failing pump—are both internal boiler components. Working on these parts requires the removal of the boiler casing, which should only be done by a qualified heating engineer. Attempting a DIY repair not only risks damaging your boiler further but can also void your warranty and, more importantly, could make your heating system unsafe.
💡 Under UK law, only a Gas Safe registered engineer is legally allowed to repair or replace internal boiler components.
✔ Step 1: Diagnose the Problem
The engineer will check the pressure sensor and pump to confirm which part is faulty.
They may manually test the pump to see if it’s generating enough pressure.
If there is sludge buildup, they might inspect the system for blockages.
✔ Step 2: Replace the Faulty Component
If the pressure sensor is faulty, they will remove the old one and fit a replacement.
If the pump is weak or broken, they will install a new one and test its performance.
If both parts are heavily worn, they may recommend replacing both for long-term reliability.
✔ Step 3: Check for System Contamination
If the engineer finds sludge or debris in the system, they may suggest a power flush to remove blockages and protect the new components from future damage.
✔ Step 4: Reset and Test the Boiler
Once the repairs are complete, the engineer will reset the boiler, test the heating system, and ensure everything is working properly before leaving.
If you ignore or delay fixing the F75 fault code, you could be putting your boiler at risk of further damage. Here’s what might happen:
⚠️ Complete Boiler Failure – If the pump completely fails, your heating and hot water will stop working entirely.
⚠️ Increased Repair Costs – The longer you leave the issue, the more strain it can put on the system, potentially causing other parts to fail.
⚠️ Wasted Energy – A failing pump or sensor can make your boiler work inefficiently, leading to higher energy bills.
⚠️ Potential for System Blockages – If sludge buildup is contributing to the fault, delaying a fix could lead to a clogged system, requiring a costly power flush
There are two main reasons why an F75 fault appears:
1️⃣ A Faulty or Blocked Pressure Sensor – If the pressure sensor is dirty, clogged, or completely broken, it won’t be able to detect pressure changes. Sludge and debris in the system can block the sensor, making it unresponsive.
2️⃣ A Faulty or Weak Boiler Pump – The pump is responsible for circulating water around the heating system. If the pump is weak or completely failed, it won’t create the pressure changes needed to activate the boiler.
🔹 Air in the system – Trapped air in the pipework or radiators can affect pressure changes.
🔹 Low boiler pressure – If the pressure is too low, the sensor may not detect any movement.
🔹 Sludge or debris buildup – Over time, dirt and sludge can clog the pump and sensor, especially in older systems without inhibitor chemicals.
🔹 Worn-out pump seals – If the pump seals are damaged, it may struggle to circulate water properly.
💡 Vaillant boilers manufactured after 2012 have improved pressure sensors, making the F75 fault less common in newer models.
Unlike some minor boiler issues that can be resolved with a quick DIY fix, the F75 fault requires professional repair by a Gas Safe engineer. This is because the issue is typically caused by either a faulty pressure sensor or a failing boiler pump, both of which involve working on internal components of the boiler, something only a qualified engineer should do.
The cost of fixing the Vaillant F75 fault depends on whether the issue is with the pressure sensor or the pump.
Free Fixes You Can Try Yourself:
Repressurising the boiler – £0
Bleeding radiators – £0
Professional Repairs (If Needed):
Fault | DIY Fix? | Estimated Cost |
Low boiler pressure | ✅ Yes | £0 |
Air in radiators | ✅ Yes | £0 |
Blocked pressure sensor (cleaning) | ❌ No | £80-£120 (cleaning or minor repairs) |
Replacing the pressure sensor | ❌ No | £100-£200 (parts & labor) |
Faulty or weak boiler pump | ❌ No | £200-£450 (depending on the model) |
System power flush (to remove sludge) | ❌ No | £300-£600 |
🔹 If your Vaillant boiler is over 10 years old, it may be more cost-effective to replace both the sensor and pump at the same time.
The Vaillant F75 fault is often caused by internal wear and tear, sludge buildup, or pressure fluctuations in the heating system. While replacing a faulty pressure sensor or pump will fix the issue in the short term, taking preventative steps will help ensure that your boiler continues running efficiently and reduce the risk of the F75 error returning.
Here’s how you can protect your boiler and avoid facing the same issue again:
A key factor in preventing an F75 fault is maintaining stable system pressure. If the pressure is too low, the boiler may struggle to detect pressure changes, leading to the same issue occurring again.
🔹 How to Check Boiler Pressure:
1️⃣ Locate the pressure gauge on the front of the boiler.
2️⃣ When the system is cold, the pressure should be between 1.0 and 1.5 bar.
3️⃣ If the pressure drops below 1.0 bar, your system may need repressurizing.
4️⃣ If the pressure rises above 2.5 bar, you may need to bleed a radiator to release excess pressure.
💡 Tip: If your boiler keeps losing pressure, there could be a small leak in the system, or the expansion vessel may need recharging. An engineer should inspect it to prevent future faults.
Trapped air in the system can interfere with pressure changes, making it harder for the pressure sensor to detect normal fluctuations when the pump starts running. This can contribute to an F75 fault, especially in older systems.
🔹 How to Bleed Your Radiators:
1️⃣ Turn off the heating and let the system cool.
2️⃣ Use a radiator key to slowly open the bleed valve (found at the top corner of each radiator).
3️⃣ If air is trapped, you’ll hear a hissing sound as it escapes.
4️⃣ When water starts to come out steadily, close the valve.
5️⃣ Check the boiler’s pressure gauge—if it has dropped, top it up using the filling loop.
💡 Tip: If your radiators are cold at the top and warm at the bottom, they likely need bleeding. Doing this every few months helps keep the system balanced and prevents unnecessary pressure issues.
Over time, rust, dirt, and mineral deposits can accumulate inside the boiler and central heating system, forming sludge. This sludge can clog the pump and pressure sensor, leading to an F75 fault.
🔹 Benefits of Using an Inhibitor:
✔ Prevents rust and corrosion in pipework, radiators, and the boiler.
✔ Stops sludge from forming, keeping the system clean and efficient.
✔ Helps to extend the lifespan of your pump and pressure sensor by preventing blockages.
🔹 How to Add Inhibitor Chemicals:
Your engineer can add inhibitor chemicals during an annual boiler service.
If you’re confident in handling your heating system, inhibitors can be added through a radiator or magnetic filter.
💡 Tip: If your system already has significant sludge buildup, a power flush may be required to clear existing debris before adding an inhibitor.
Many boiler faults, including F75, can be prevented by having an annual service. A Gas Safe engineer will inspect and clean key components, helping to prevent issues before they cause a breakdown.
🔹 During a Boiler Service, an Engineer Will:
✔ Test the pump to ensure it’s functioning correctly.
✔ Inspect and clean the pressure sensor to prevent false readings.
✔ Check for leaks or signs of pressure loss.
✔ Top up inhibitor chemicals and ensure the system is well-protected.
💡 Tip: Booking a boiler service before winter ensures your heating system is working efficiently when you need it most.
A magnetic filter is one of the best long-term investments for your heating system. It traps metal debris, rust particles, and sludge, preventing them from entering the boiler and damaging components like the pump and pressure sensor.
🔹 Why a Magnetic Filter Helps:
✔ Prevents clogging of the pump and pressure sensor, reducing the risk of an F75 fault.
✔ Helps to maintain efficiency by keeping the water flowing freely through the system.
✔ Reduces the need for expensive repairs caused by sludge buildup.
💡 Tip: If you don’t already have a magnetic filter, ask your Gas Safe engineer to install one during your next boiler service.
✔ Run the heating regularly – Even in summer, turning on the heating for 10-15 minutes every few weeks helps prevent the pump from seizing.
✔ Check for small leaks – Even minor drips around radiators or pipe joints can cause pressure issues over time.
✔ Ensure the filling loop is fully closed – If left slightly open, it can lead to gradual pressure loss, affecting pump performance.
If your Vaillant boiler is displaying the F75 fault code, the only solution is to call a Gas Safe registered engineer to diagnose and repair the issue properly. Attempting to fix it yourself is not safe or recommended, as it involves working on internal boiler components.
🚨 Don’t risk further damage or an unsafe heating system—book a Gas Safe engineer today to get your boiler working again!
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