What the F75 code means
Each time the pump starts, the boiler expects to see a small jump in water pressure as the pump pushes water round the system. The pressure sensor watches for that jump. On most ecoTEC models, if the boiler does not see it after several tries, it assumes something is wrong with circulation and locks out with F75.
That means F75 is really a question the boiler cannot answer: is the pump not working, is the sensor not reading, or is there not enough water or too much air for the pump to build pressure? Some of those you can rule out yourself. Most of the real fixes need a Gas Safe registered engineer, which is why we class F75 as an engineer fault.
F75 often shows up when the heating first comes on after a quiet spell, for example on the first cold morning of autumn. That timing is a useful clue. A pump that has sat idle for months is more likely to stick, and air has had time to collect in the system.
Common causes of F75
Faulty or seized pump
Pumps wear out, and they can also stick after a long period of not running, such as over the summer. A pump that hums but does not turn, or makes no sound at all, will not create the pressure rise the boiler is looking for.
Faulty pressure sensor
If the sensor itself has failed, or its wiring has a poor connection, it may not register a pressure change even when the pump is working. This is one of the most common F75 repairs.
Air in the system
Air trapped in the pump or pipework stops the pump moving water properly. This often happens after work on the heating system, after draining down, or after a big pressure drop.
Low system pressure
If pressure is very low, the pump may not be able to build a noticeable rise. This overlaps with the F22 low pressure fault, and sometimes one code follows the other.
Expansion vessel problems
The expansion vessel absorbs changes in pressure. If it has lost its air charge or failed, the pressure behaviour of the system changes and can confuse the reading when the pump starts.
Sludge and blockages
In older systems, magnetite sludge can build up in the pump and pipework and restrict flow. Cold spots at the bottom of radiators are a common sign.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Low system pressure | Gauge below 1 bar when cold | You, via the filling loop |
| Air in the radiators | Radiators cold at the top, gurgling | You, by bleeding radiators |
| Air in the pump | Pump noisy or not circulating | Gas Safe engineer |
| Faulty pressure sensor | Pump runs but F75 persists | Gas Safe engineer |
| Faulty or seized pump | No circulation, humming or silence | Gas Safe engineer |
| Expansion vessel or sludge | Pressure swings, cold spots on radiators | Gas Safe engineer |
Safe checks you can do yourself
- Check the pressure gauge with the heating off and cool. If it is below 1 bar, top up to 1 to 1.5 bar using the filling loop. Our guide on how to repressurise a boiler explains each step.
- If radiators are cold at the top or you can hear gurgling, bleed them with the heating off, then check the pressure again and top up if needed.
- Make sure the filling loop valves are fully closed afterwards.
- Check the fused spur and power to the boiler are on.
- Press reset as your manual describes. Stop after two or three attempts.
Do not remove the casing, try to free the pump or bleed air from the pump yourself. The pump sits inside the boiler next to live electrical parts and gas components, so it must be handled by a Gas Safe registered engineer.
When to call a Gas Safe engineer
If the pressure is correct, the radiators are bled and F75 still comes back, call a Gas Safe registered engineer. You should also call one if you can hear the pump humming without circulating, or if the pressure keeps dropping. The engineer will usually test the pump, check the pressure sensor and its wiring, release any trapped air from the pump, and check the expansion vessel charge. In older systems they may also suggest cleaning the system to remove sludge.
What to tell the engineer
When you book, give the exact code, the reading on the pressure gauge, and whether you topped up or bled radiators before the code appeared. Mention any noises from the boiler, such as humming, gurgling or knocking. This helps the engineer bring the right parts, as pumps and pressure sensors differ between ecoTEC models.
Typical repair costs in the UK
F75 repairs usually involve a part. These ranges are typical, but your final bill depends on the model, the parts needed and your area.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Pressure sensor replacement | Typically £120 to £220 |
| Pump replacement | Typically £250 to £450 fitted |
| Expansion vessel recharge | Typically £80 to £150 |
| Magnetic filter fitted | Typically £150 to £250 |
| System clean or powerflush | Typically £400 to £700 |
How to stop F75 coming back
Keep the system pressure between 1 and 1.5 bar when cold and check it monthly. Run the heating briefly once a month over the summer so the pump does not stick. If your system has no magnetic filter, ask your engineer whether one is worth fitting, as it helps protect the pump from sludge. A yearly service is also a good chance to check the expansion vessel. Our boiler service cost guide explains what is normally included.