Of all the fault codes Vaillant ecoTEC owners ring me about, F75 is the one most likely to be misdiagnosed by another engineer. Why? Because the easy thing to do is to swap the pressure sensor (a £65 part), charge the customer £180, and walk away. Two weeks later the F75 is back because the sensor wasn't the problem in the first place.
This guide walks through the real diagnosis path Vaillant-trained engineers use, in order of probability. If you ring me for an F75, this is exactly the order in which I check things.
What F75 actually means
F75 is Vaillant's safety logic responding to a specific scenario: the pump has started, but the boiler hasn't detected a rise in water pressure within the first few seconds.
Think about it from the boiler's perspective. When you turn the heating on, the pump kicks in, water starts moving, and the system pressure normally jumps slightly because of thermal expansion. The boiler watches the pressure sensor: if the sensor reading doesn't tick up at all within the expected window, the boiler concludes one of three things:
- The pump is running but no water is moving (pump airlock, stuck impeller, or restriction).
- The pump isn't actually running (electrical fault, seized motor).
- The pressure sensor itself is broken and isn't reporting what's actually happening.
Rather than risk overheating the heat exchanger, the boiler immediately shuts down the burner and flashes F75. It's a sensible safety move — but it doesn't tell you which of those three is the cause.
The diagnosis order (do these in sequence)
Step 1 — Is your pressure gauge above 1.0 bar?
If it's below 1.0 bar, you actually have low pressure, not F75 as such. Repressurise to 1.2 bar via the filling loop (same procedure as the Worcester F22 fix — see the F22 guide). If after repressurising the F75 clears, you're done. If F75 is still showing with the gauge at 1.2 bar, continue to step 2.
Step 2 — Can you hear the pump running?
Press your ear to the boiler front while it's trying to fire (it'll usually attempt a re-ignition every 2-3 minutes). You should hear a low electrical hum from inside the casing — that's the pump motor. If you hear it: you have a pump airlock or a sensor issue (continue). If you hear nothing: you may have a seized pump motor or electrical fault — call.
Step 3 — Try the pump unstick trick (homeowner-safe)
// The pump unstick
- Switch the boiler to standby (not off). Press the standby button; leave the electrical spur on.
- Wait 5 minutes. Lets the pump cool and any air bubbles rise to the top of the pump head.
- Bleed every radiator. Start with the downstairs ones, working your way up. Use a bleed key, turn each valve until a small hiss of air comes out, then a dribble of water — close it quickly. You're chasing air out of the system.
- Top up the pressure to 1.2 bar. Air leaving the system drops the pressure.
- Reset the boiler. Hold the reset button for 5 seconds. Listen for the burner to light within 30 seconds.
If this works (and it does about half the time), congratulations — you've just saved yourself £180. The reason the trick works is that air bubbles in the pump head break the impeller's "grip" on the water, so the pump spins without moving water, and the boiler can't detect a pressure rise.
Some YouTube videos tell you to remove the slotted silver screw on the front of the pump head to release air. This is a Gas Safe job — the pump is in the same enclosure as the gas valve and the heat exchanger. Bleeding the pump from outside the casing (via the radiators) is the homeowner-safe equivalent and works in most cases.
If the unstick trick didn't work
Three possibilities now, in order of likelihood:
1. The pressure sensor is failing intermittently (20% of cases)
The sensor on a Vaillant ecoTEC sits at the bottom of the boiler, between the pump and the heat exchanger. They typically fail in two ways: gradually (the resistance drifts out of calibration, so the boiler thinks pressure isn't rising even when it is) or intermittently (works when warm, fails when cold — classic Wembley-winter-morning F75 pattern). A new sensor is £65 of parts and 45 minutes of labour. Total fitted: £140-180 in Wembley.
2. The pump itself has gone (15% of cases)
Vaillant ecoTEC pumps are Grundfos units; they're well-built but they don't last forever. Bearings wear, impellers slowly chip. If the unstick trick works for 2-3 days and then F75 returns, the pump is on its way out. Total fitted: £280-380.
3. The system is sludged and the pump is straining (5% of cases)
If your boiler is more than 8 years old, has never been power flushed, and the F75 is paired with cold patches on radiators, you've probably got iron-oxide sludge restricting flow. The pump spins, but water moves so slowly that pressure doesn't rise enough to satisfy the sensor. A full system power flush plus inhibitor refresh is £395-495 and usually solves it for another 8-10 years.
Tried the unstick? Still F75?
I cover all HA & NW postcodes. Diagnosis on the same visit, fixed quote before parts come out of the van. Vaillant-trained, parts on the van.
Vaillant F75 cost reference (Wembley 2026)
The trap: why some engineers always swap the sensor first
It's tempting. The sensor is cheap (£65 part), easy to access, and changing it has a high "looks-like-progress" feel. Some engineers will swap it as a default, even before checking whether the pump runs or whether the system is sludged. If the actual cause was a sludged system, the F75 will be back within two weeks — usually the day before Christmas.
The give-away that you've had a "swap-the-sensor-and-leave" engineer: they didn't run a flue gas analyser test on the way out, didn't write down the pre- and post-flow temperatures, and didn't suggest a system inhibitor top-up. A proper diagnosis on F75 takes 45 minutes minimum.
Frequently asked questions
What does F75 mean on a Vaillant boiler? +
F75 means the boiler started the pump but didn't detect a rise in water pressure within a few seconds. The boiler stops the burner to protect the heat exchanger. Causes are an air lock, a stuck pump, a failed pressure sensor or sludge restriction.
Can I fix Vaillant F75 myself? +
Sometimes. Bleeding every radiator, repressurising to 1.2 bar and resetting the boiler clears about 50% of cases. The remaining 50% need a Gas Safe engineer for a sensor, pump or power flush.
How much does it cost to fix a Vaillant F75 fault? +
£140–£180 for a pressure-sensor replacement, £280–£380 for a full pump replacement, £395–£495 for a power flush if the underlying issue is sludge.
Why does F75 keep coming back? +
Three reasons: the system is sludged and the pump is straining; the pressure sensor is failing intermittently; or there's a slow leak elsewhere causing pressure to drop just enough to trigger the fault on every restart.
Is F75 dangerous? +
No — F75 is the boiler protecting itself by refusing to fire. The danger would be if the boiler kept firing without water flow. As long as the boiler is showing F75 and not running, it's safe. Don't override the lockout.
Written by Jamil — Gas Safe registered engineer (Reg. 930067), based in Wembley HA9. Vaillant-trained for the ecoTEC Pro, Plus and Exclusive ranges. Last updated 29 April 2026.