Glow-worm F75 fault codewhat it means and how to fix it
The boiler saw no pressure rise when the pump started, so it locked out. Almost always the pressure sensor or the pump, not low pressure.
Take care
Is the pump running?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
No pump noise at all
Pump failure or no signal reaching it. Either way it is inside the case.
Book an engineer rather than resetting.
The pump runs but it still locks out
Most often a failed pressure sensor — an inexpensive part and a short job.
Mention that the pump is audibly running; it saves diagnostic time.
It started right after the system was worked on
Air lock. Trapped air stops the pump generating any pressure change.
Call whoever did the work back to vent it properly.
What F75 means
F75 catches people out because it looks like a pressure fault and is not. When the boiler starts a heating cycle it runs the pump and expects the pressure sensor to register a small rise as circulation begins. If that rise never appears, the board concludes either the pump is not moving water or the sensor is not reading it, and it locks out. The absolute system pressure can be perfectly correct throughout. That is why topping the system up does nothing for F75, and why people spend a weekend refilling a boiler that never had a pressure problem in the first place.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor silted with magnetite | Glow-worm mount the transducer where debris naturally settles. Once coated it reports a believable static figure and stops responding to change, which is exactly the condition this code detects. | Common |
| Pump not shifting water | Seized after a summer idle, or worn to the point where it turns without producing useful flow. Frequently arrives with radiators that never quite get hot. | Common |
| Air standing in the pump body | Common after a drain-down, a radiator swap, or a refill done at speed. The impeller spins in air and generates nothing for the sensor to see. | Occasional |
| Broken wiring between sensor and board | An open circuit reads the same as a dead sensor and costs a fraction as much. Worth eliminating before parts are ordered. | Occasional |
What to try yourself
Read the gauge cold before anything else. A normal 1.0 to 1.5 bar alongside a persistent F75 is the confirmation you need that this is not a low-pressure fault, and it will stop you wasting a weekend refilling.
Work round the radiators bleeding each one, ground floor first and the highest last, then bring the pressure back to 1.2 bar afterwards. Trapped air is the one cause on the list you can clear yourself.
Put a hand on the pipework under the appliance while it tries to start. Warmth spreading within a minute means water is moving; a silent, cold pipe points squarely at the pump.
Reset once. When you book, say the pressure gauge reads normal — on a Glow-worm that single sentence sends the engineer to the transducer rather than round the houses.
Do not keep resetting. Forcing an appliance to fire with no proven circulation is how heat exchangers get destroyed.
Get a Gas Safe registered engineer to look at your Glow-worm F75
Gas Safe Register engineers only. No obligation.
What it costs to fix
Typical repair cost
£140–£450
Time on site
1 hour
Pressure sensor £140–£220 fitted. Pump £280–£450. System clean, if needed, £400–£700.
Typical UK prices including parts and labour, updated August 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant ecoTEC water pressure sensor
0020059717£26–£54Authorised Vaillant parts distributor
Price checked 5 August 2026
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 5 August 2026
What other owners found
Nobody has shared what worked for them on this code yet. If you have had it, you would be the first — and it genuinely helps the next person.
Related Glow-worm fault codes
- F22Get it checkedThe boiler has locked out because system pressure is too low to run safely. Topping up to 1.2 bar and resetting normally fixes it.Related: water pressure
- F.9Get it checkedA circulation or pressure sensing fault. Check the pressure and open all radiator valves before assuming the worst.Related: water pressure
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: water pressure
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: water pressure
- F1Get it checkedSystem pressure has dropped below the minimum the boiler will run at. Topping it back up to 1.2 bar usually clears it straight away.Related: water pressure
- CE 207Get it checkedSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Related: water pressure
Related guides
Common questions about Glow-worm F75
Will topping up the pressure fix F75?
No, and this is the single most common wasted afternoon with this code. F75 is about the change in pressure when the pump runs, not the absolute value. A boiler sitting at a perfect 1.2 bar throws F75 all day long.
What does the pressure sensor cost?
The part is £25 to £50 and the job is usually under an hour, so most people pay £140 to £220 all in.
Why does it clear sometimes and come back?
A sensor on the way out works intermittently before it fails completely, and an air lock can shift as the system circulates. Either way it is a fault in progress, not one that fixed itself.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Interviews with working engineers
Checked by Technical reviewer (Gas Safe registration to be displayed on appointment) on 5 August 2026.