F75 on a Vaillant boiler means the boiler's control board has failed to detect a pressure change when the pump starts up. The boiler expects to see a small rise in pressure the moment the pump activates - if that change is not detected within the expected window, the boiler locks out and displays F75. The three causes are a faulty pressure sensor, a failed or seized pump, or a significant leak in the heating system that is preventing normal pressure build-up. Unlike the F22 fault, F75 cannot be resolved by repressurising the system - it requires a Gas Safe registered engineer to diagnose and repair the underlying component fault.
What Does F75 Mean on a Vaillant Boiler?
The F75 fault code is a safety lockout that appears on Vaillant ecoTEC, ecoTEC Plus, ecoTEC Pro and ecoFIT Pure boilers. It is triggered when the boiler's control board starts the pump as part of a normal heating or hot water demand cycle - but the pressure sensor fails to detect the expected rise in water pressure that should follow.
To understand why this matters, it helps to know what should happen during a normal pump start. When the pump activates, it pushes water through the sealed heating circuit, creating a small but measurable increase in system pressure. The boiler's pressure sensor monitors this change. If it registers the expected pressure change, the boiler confirms the pump is working correctly and proceeds with the heating cycle. If no pressure change is detected within the expected window, the boiler concludes that either the pump has failed to move water, or the sensor cannot read the change - and it locks out with F75 to prevent damage to the heat exchanger from running without adequate water flow.
This is a fundamentally different fault from F22. F22 tells you the system does not have enough water pressure. F75 tells you the boiler cannot detect what the water pressure is doing when the pump runs. The system may have perfectly normal pressure on the gauge and still show F75 if the sensor or pump has failed.
F75 vs F22: Understanding the Difference
These two fault codes are frequently confused because both involve boiler pressure. The distinction is critical - they have completely different causes and require completely different responses.
| Aspect | F75 Pump/Sensor Fault | F22 Low Pressure Fault |
|---|---|---|
| What it means | No pressure change detected when pump starts | System water pressure is too low |
| Pressure gauge reading | Often shows normal (1.0-1.5 bar) | Shows low (typically below 1.0 bar) |
| Primary cause | Failed pump, faulty sensor, or major leak | Low water, slow leak, or expansion vessel fault |
| Can homeowner fix? | No - Gas Safe engineer required | Often yes - repressurise via filling loop |
| Will repressurising help? | No - topping up will not clear F75 | Yes - usually clears a first-time F22 |
| Urgency | Book engineer promptly - boiler is offline | Repressurise first; call engineer if it recurs |
Three Causes of the Vaillant F75 Fault Code
F75 has three possible root causes. A Gas Safe engineer will work through these systematically to identify the fault. Understanding what they are looking for helps you have a more informed conversation when the engineer arrives - and helps you understand what is likely driving the cost of the repair.
1. Faulty Water Pressure Sensor (Most Common)
The pressure sensor is a small electronic component that monitors system pressure and reports changes to the boiler's control board. Over time - particularly in hard water areas such as London - limescale and debris accumulate around the sensor port, gradually impairing its ability to detect pressure changes accurately. The sensor physically cannot register the pressure rise when the pump activates and reports no change to the control board, triggering F75. The pump may be working perfectly. If the boiler produces a rumbling vibration when the pump runs but still shows F75, a blocked or faulty sensor is the most likely diagnosis. Sensors can sometimes be cleaned, but replacement is usually more reliable and cost-effective than cleaning alone.
Gas Safe engineer required2. Failed or Seized Pump
The pump circulates heated water through the radiators and pipework. If the pump has seized - either from a lack of use (common in boilers that have been idle over summer), bearing wear, or sludge build-up in the heating system - it cannot move water when it activates. No water movement means no pressure change, and the boiler locks out with F75. Early Vaillant ecoTEC models manufactured before approximately 2005 were fitted with pumps that were particularly prone to failure and were later redesigned. If there is no vibration or sound from the boiler when the pump should be running, a seized or failed pump is the primary suspect. An engineer will attempt to free a seized pump before recommending replacement - but a pump that has failed due to age or sludge damage will need replacing. A full system power flush is often recommended alongside pump replacement to remove the debris that caused the failure.
Gas Safe engineer required3. A Significant Leak in the Heating System
If the heating system has a significant leak - one that is large enough to prevent normal pressure build-up when the pump activates - the boiler may display F75 rather than F22. This is because the system cannot maintain the pressure that the sensor expects to detect during pump operation. The water is escaping as fast as, or faster than, the pump can circulate it. This is a less common cause of F75 than a sensor or pump fault, but it should always be considered if the sensor and pump are found to be functioning correctly. A concealed leak beneath a floor, within a wall cavity or under a concrete screed can cause this level of pressure instability without any visible water at the surface. Specialist central heating leak detection is required to locate the fault without unnecessary disruption to the property.
Leak detection specialist requiredHow to Diagnose the Cause of F75: What the Engineer Will Do
When a Gas Safe engineer attends an F75 fault, they follow a logical diagnostic sequence to identify which of the three causes is responsible. Understanding this process helps you know what to expect during the visit and why the engineer may need to run the boiler through several test cycles before confirming the diagnosis.
Check the System Pressure Gauge
The first check is the physical pressure gauge on the boiler. If it reads below 0.5 bar, the system needs repressurising before any further diagnosis - a severely depressurised system can cause F75 symptoms. If the pressure reads within the normal range (1.0-1.5 bar) and F75 is still present, the fault is almost certainly with the pump or sensor rather than system pressure. This is where F75 diverges sharply from F22.
Listen and Feel for Pump Operation
The engineer resets the boiler and initiates a demand cycle - either a hot water or heating call - while listening carefully. A functioning pump produces a distinct low rumble or vibration, typically felt at the boiler casing. If the rumble is present and the boiler still locks out with F75, the pump is working but the sensor is not detecting the pressure change - pointing to a sensor fault. If no rumble or vibration is detectable, the pump is not activating or is seized - pointing to a pump fault.
Inspect and Test the Pressure Sensor
The engineer accesses the pressure sensor - usually located on the hydraulic manifold inside the boiler - and checks for visible limescale or debris blocking the sensor port. They may use diagnostic equipment to test the sensor's electrical output and compare it against the expected signal range. A sensor that is producing no signal or an erratic reading confirms a sensor fault. In some cases, the engineer will temporarily substitute a known-good sensor to confirm the diagnosis before ordering a replacement.
Inspect and Test the Pump
If the sensor appears functional, the engineer inspects the pump. They check whether the pump shaft is turning freely - a seized pump shaft is a definitive diagnosis. On some models, a pump bleed screw can be loosened to check for water flow. The engineer may also check the pump capacitor, which can fail and prevent the pump from starting even when the motor itself is intact. Pump age, visible corrosion, and system sludge levels all inform whether repair or replacement is the appropriate course of action.
Rule Out a Concealed Leak (If Pump and Sensor Are Sound)
If both the sensor and pump test as functional, the engineer considers whether a significant leak elsewhere in the heating circuit could be causing the pressure behaviour that triggers F75. This may involve a broader pressure test of the circuit. If a concealed leak is suspected, referral to a specialist central heating leak detection survey is the appropriate next step - standard plumbing tools cannot locate leaks hidden beneath floors or inside walls without causing unnecessary damage.
What Happens Once the Cause Is Confirmed?
The repair path depends entirely on which component has failed. Here is what each diagnosis typically means in practice.
Pump Replacement
A seized or failed pump is removed and replaced. The engineer will typically recommend a system power flush to remove the sludge that contributed to pump failure before fitting the new unit. A magnetic filter installation is also advised to catch future debris before it reaches the pump.
Sensor Replacement
The pressure sensor is a relatively straightforward component replacement. Once the correct sensor for the boiler model is fitted and the system is bled and repressurised, the boiler should restart and clear the F75 fault. Sensor replacement is typically the lower-cost F75 repair.
Leak Detection Survey
If a concealed heating leak is identified as the cause, a specialist engineer uses thermal imaging, tracer gas or acoustic detection to pinpoint the fault without lifting floors unnecessarily. A written report is produced for insurance purposes.
Boiler showing F75 and engineer suspects a hidden leak? WaterLeakFinder connects London homeowners with specialist engineers who locate concealed heating leaks without unnecessary floor damage.
Get HelpWhen F75 Points to a Hidden Heating Leak
Of the three causes of F75, a concealed heating system leak is the least common - but it is also the most damaging if left unaddressed. When an engineer confirms that both the pump and pressure sensor are functioning correctly yet F75 persists, the focus shifts to whether the heating circuit itself is losing water at a rate that prevents normal pressure behaviour during pump operation.
Central heating pipework in London properties frequently runs beneath concrete floors, within screed beds, inside partition walls and under timber subfloors. A pinhole corrosion leak, a failed push-fit joint, or a weeping soldered connection in any of these concealed locations can drain pressure from the circuit faster than the pump can build it - producing the pressure behaviour that triggers F75. The leak may not produce any surface moisture visible to the occupant.
This matters for two reasons. First, a concealed heating leak causes ongoing structural damage - saturating timber joists, destabilising screed, promoting mould growth in wall cavities and corroding reinforcement in concrete construction - entirely invisibly, while the boiler continues to lock out. Second, a leak of this scale in a concealed location requires specialist non-invasive detection methods to locate accurately.
Signs That F75 May Be Caused by a Concealed Heating Leak
- Both the pump and pressure sensor have been tested as functional by an engineer
- F75 returns immediately or very soon after repair of sensor or pump
- Pressure gauge drops noticeably between boiler lockout cycles
- Damp patches, staining or soft spots in the floor near radiator pipework
- Blistering paint, rising damp or a musty smell near skirting boards or walls
- Sound of running or dripping water with no visible source
- Unusually high water meter readings with no other obvious cause
- Neighbouring properties have experienced similar concealed leaks
WaterLeakFinder connects London property owners with specialist engineers who carry the non-invasive equipment required to locate concealed central heating leaks. Thermal imaging reveals the heat footprint of leaking water beneath floor surfaces. Tracer gas - a safe, inert mixture injected into the pipe under pressure - rises through concrete, screed and floor coverings and is detected at the surface with a sensitive probe. Acoustic detection identifies the sound signature of pressurised water escaping from a concealed fault point.
All detection surveys produce a written report suitable for Trace and Access insurance claims. Many UK home insurance policies include Trace and Access cover - check your policy before commissioning a survey, and ask your insurer whether they require a specialist detection report before approving a claim. See our guide to central heating leak detection in London for more detail on what a survey involves.
Preventing F75 Faults: Boiler and System Maintenance That Reduces Risk
Many F75 faults are preventable. The most common causes - sensor blockage from limescale and pump failure from system sludge - both result from inadequate ongoing maintenance of the heating system. The following steps significantly reduce the likelihood of a pump or sensor fault developing.
Annual Boiler Service
A Gas Safe engineer carrying out an annual boiler service will inspect the pressure sensor for limescale build-up, check the pump for correct operation, test the hydraulic components and flush the primary heat exchanger. Most F75-related faults are identified and resolved at a service before they cause a full breakdown. An annual service is particularly important for Vaillant boilers in London, where hard water accelerates limescale accumulation inside the boiler.
Install a System Filter
A magnetic system filter - fitted in line with the heating circuit return - captures iron oxide sludge and debris before it reaches the pump and other hydraulic components. Sludge accumulation is the primary cause of pump failure in central heating systems. A filter significantly extends pump life and reduces the risk of F75 from pump seizure. The filter canister should be cleaned annually as part of the boiler service.
Consider a Power Flush
If your heating system is more than 10-15 years old and has never been flushed, there may be significant sludge accumulation inside the pipework and pump. A power flush - in which a specialist machine circulates cleaning chemicals through the circuit at high velocity - removes accumulated debris and protects pumps, sensors and the heat exchanger. A power flush is particularly valuable before fitting a new boiler or pump, as new components introduced into a contaminated system will fail prematurely.
Use Inhibitor Fluid
Central heating inhibitor is a chemical added to the heating circuit water that slows corrosion and prevents sludge formation. It should be present in every sealed heating system. The concentration diminishes over time and should be checked and topped up annually. An engineer can test the inhibitor level with a simple kit during the service visit.
Do Not Ignore a Recurring F75
A boiler that keeps displaying F75 - even after a sensor or pump has been replaced - is signalling an unresolved underlying problem. Repeatedly resetting the boiler without addressing the root cause risks damage to the heat exchanger, which is the most expensive component in the boiler to replace. A recurring F75 after component replacement should prompt an engineer to reconsider whether a concealed heating leak is contributing to the fault.