Protherm F.75 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.75 fault code mean?
The F.75 fault code appears when the boiler's pressure sensor fails to detect any change in water pressure after the circulation pump has started. Under normal conditions, the pump kicks in and creates a measurable shift in pressure as water begins to move around the system. If the PCB receives no confirmation that this shift has occurred — whether because the sensor is faulty, the pump isn't working, or the system pressure is too low — the boiler locks out and displays F.75 as a safety precaution. Without confirmed water movement, the heat exchanger could overheat and sustain serious damage. It's worth noting that Protherm is part of the Vaillant Group, so the F.75 code carries an identical meaning across Protherm, Vaillant, and Glow-worm boilers.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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
- Blocked or faulty pressure sensor Common
The pressure sensor has a tiny port through which it reads system pressure. Over time this port can become clogged with heating sludge, limescale, or debris — preventing the sensor from detecting the pressure rise when the pump starts. If you can hear a faint rumbling or gurgling from the boiler every 30 seconds or so, this is a strong clue that the pump is running but the sensor is failing to register it. This is the most frequently reported root cause of F.75 on Protherm appliances.
- Seized or underperforming circulation pump Common
The pump is responsible for pushing water around the heating circuit. If the pump has seized (often due to a long period of inactivity, corroded bearings, or an airlock), it won't generate the pressure change the sensor is looking for. In this scenario the boiler is typically silent when it should be running — you won't hear the usual low hum of water movement. A pump running at the wrong speed setting can also cause the same symptom.
- Low system water pressure Common
If the system pressure has dropped below approximately 0.5 bar, there may not be enough baseline water in the circuit for the sensor to detect any meaningful change when the pump activates. This is commonly caused by small leaks, recent radiator bleeding without subsequent top-up, or a failing expansion vessel. Check the pressure gauge: for most Protherm boilers the correct operating pressure when cold is 1–1.5 bar.
- Water leak in the system or pipework Sometimes
A leak anywhere in the heating circuit — at radiator valves, pipe joints, or within the boiler itself — can reduce system pressure to the point where the sensor cannot detect a pressure change. Unlike a simple low-pressure scenario, a leak means the pressure will keep dropping after you top up the system. Look for damp patches near radiators, pipework, or under the boiler.
- Faulty PCB (printed circuit board) Rare
The PCB governs all electronic communication within the boiler. If it is damaged or develops a fault, it may fail to correctly interpret the signals coming from the pressure sensor, or fail to send the right signals to the pump — resulting in F.75 even when the sensor and pump are themselves in good working order. PCB failure tends to be a diagnosis of exclusion once all other components have been checked.
How to fix it
- Check the system pressure gauge DIY safe
Locate the pressure gauge on your boiler — it may be a physical dial or a digital readout depending on your model. The pressure should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, low pressure is likely contributing to the fault and you should top it up before anything else.
- Repressurrise the system via the filling loop if pressure is low DIY safe
Find the filling loop underneath the boiler — it consists of a short braided hose with two valves. Open one valve slowly, then the other, and watch the gauge climb. Stop when the pressure reaches 1–1.5 bar, then close both valves firmly. If you cannot locate the filling loop or are unsure, consult your boiler's user guide or call an engineer.
- Listen to the boiler after resetting DIY safe
Press the reset button and listen carefully. A rumbling or pulsing sound every 30 seconds or so suggests the pump is running but the pressure sensor is not responding — pointing to a sensor issue. Complete silence when the boiler should be firing suggests the pump may not be working at all. Pass this information on to your engineer as it will help them diagnose faster.
- Reset the boiler (up to 2–3 times) DIY safe
Press and hold the reset button for the duration specified in your user manual (typically 3–5 seconds). If the boiler restores and runs normally, monitor it over the next hour. If F.75 returns promptly, do not keep resetting — repeated lockouts with no fix indicate an underlying fault that needs professional attention.
- Check for visible leaks around the system DIY safe
Inspect the pipework, radiator valves, and the area directly beneath the boiler for any signs of moisture, dripping, or staining. If you find a leak, do not attempt to repair it yourself — note the location and report it to your engineer. If the system pressure drops again shortly after you top it up, that is itself a strong indicator of a leak.
- Arrange for a Gas Safe registered engineer to inspect the pressure sensor and pump Gas Safe engineer
If the fault persists after the above checks, a qualified engineer will need to investigate internally. The most common fix is removing the pressure sensor, clearing the blockage from its small inlet port, and refitting or replacing it — a relatively straightforward job for an experienced engineer. They will also check the pump's operation, speed settings, and whether an airlock is preventing circulation.
- Have the engineer assess the pump if sensor work does not resolve the fault Gas Safe engineer
If the sensor is confirmed to be working correctly, attention shifts to the circulation pump. The engineer may free a seized pump, clear an airlock, or replace the pump entirely if it has failed mechanically. Do not attempt to access or operate the pump yourself — it is an internal boiler component.
- Ask the engineer about a powerflush if sludge is suspected Gas Safe engineer
If sludge or debris is thought to have caused the sensor blockage or pump seizure, a powerflush of the whole heating circuit may be recommended. This clears the system of corrosion, magnetite, and debris that would otherwise cause recurring faults. It also protects other components from premature wear.
- PCB replacement as a last resort Gas Safe engineer
Should the sensor and pump both test as functional, the engineer may suspect the PCB. PCB replacement is always a job for a Gas Safe registered engineer and tends to be one of the more expensive repairs — consider the age of the boiler and whether replacement is a more cost-effective option.
Parts you may need
- Water pressure sensor · from £35
- Circulation pump · from £90
- 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 fix the Protherm F.75 fault myself?
There are a couple of checks you can safely carry out yourself: confirming the system pressure is between 1 and 1.5 bar, topping it up via the filling loop if it has dropped, and resetting the boiler. If those steps clear the fault and it does not return, you may not need an engineer at all. However, anything beyond that — cleaning or replacing the pressure sensor, working on the pump, or inspecting the PCB — must be carried out by a Gas Safe registered engineer. It is both unsafe and illegal for an unqualified person to work on the internal components of a gas appliance.
How much does it cost to fix the F.75 fault on a Protherm boiler?
For most homeowners the repair falls somewhere between £120 and £350 including parts and labour. Repressurising the system is at the lower end (sometimes done as part of a general service call). Cleaning or replacing the pressure sensor typically costs £150–£250 all in. Pump replacement tends to be the most expensive common repair, usually £250–£350. A PCB replacement can exceed £400–£500 and is worth weighing against the boiler's age — if your boiler is over 10–12 years old, a new boiler may represent better value.
Why does the F.75 fault keep coming back after I reset the boiler?
A fault that clears on reset but returns within minutes or hours almost always points to an underlying component problem rather than a one-off glitch. The most likely culprits are a pressure sensor that is partially blocked (it reads correctly sometimes but not others), a pump that is struggling intermittently, or a slow leak that keeps dropping the system pressure. Repeated resetting without addressing the root cause can accelerate wear on other components, so arrange for an engineer to investigate if the fault returns more than twice.
Does the F.75 fault mean the same thing on Vaillant and Glow-worm boilers?
Yes. Protherm is owned by the Vaillant Group and shares its core engineering platform with Vaillant and Glow-worm. The F.75 code on a Protherm boiler is directly equivalent to the F.75 fault on a Vaillant boiler and the F.75 on a Glow-worm — all indicate that no pressure change was detected after pump start. The causes and fixes are identical across all three brands, and many parts are interchangeable.