F75 Vaillant
Vaillant F75 Fault: Pump / Pressure Sensor Explained
F75 is Vaillant's best-known character fault: on most ecoTEC models it means the boiler started its pump but didn't detect the expected pressure change — so either the pump didn't actually move water, or the water pressure sensor didn't register that it had. It's a known pattern on certain ecoTEC generations, well understood and routinely fixed.
What the F75 code means
At each start, the ecoTEC runs a self-check: pump on, and the internal water pressure sensor must see the small pressure rise that proves water is moving. F75 records that proof failing. Two honest suspects share the code by construction — a pump that isn't pumping (seized, degraded, air-locked) and a sensor that can't see (its sensing port gummed by system debris, a genuinely common mode on affected generations). The design's virtue is refusing to fire a burner over unmoving water; its quirk is a code that names two components at once.
What you can safely do yourself
With sensor-family codes the homeowner playbook is short: verify the readable basics (pressure gauge in the green; no dripping from the external relief pipe; heating water not doing anything visibly odd), press reset once, and observe. Note whether the boiler runs at all, for how long, and what the display cycles through — the pattern is genuinely diagnostic and worth reporting when you book.
What resolves these codes is a multimeter, the model's service manual and often a modestly priced replacement sensor — an unremarkable visit for an engineer who sees the brand weekly. What doesn't resolve them is repeated resetting: if the boiler's instrumentation says something is wrong, either the instrument or the something needs professional attention, and both live behind the case.
What causes it
The famous cause first: the water pressure sensor's port fouling with system debris — magnetite fines in imperfectly clean systems settle exactly there, and the sensor goes progressively blind. Then: pump degradation or seizure (summer-idle pumps sticking at the season's first call is the classic timing), air locks after system work, genuinely low system pressure compounding the proof, and wiring or connector faults in the tail. The system-water subtext is real: F75 recurrence on a sludgy system is the sensor reporting the water quality as much as any component.
When to call a Gas Safe engineer
F75 is engineer work with a well-worn playbook: verify pressure and purge air, test the pump, inspect or replace the sensor — both parts are modestly priced and stocked everywhere the brand is common. Book on first occurrence rather than resetting through it; the failure modes don't self-heal. And ask the attending engineer to check inhibitor levels while there: the sensor-fouling mode is a system-cleanliness messenger, and treating the water is what stops the sequel.
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Frequently asked questions
What does F75 mean on a Vaillant boiler?
Pump or water pressure sensor fault: the pump ran but the boiler didn't detect the pressure change proving water moved — so either the pump isn't pumping or the sensor can't see. Both are routine, modestly priced fixes.
Why is F75 so common on ecoTECs?
A known pattern on certain generations: the pressure sensor's port fouls with system debris and goes blind, and summer-idle pumps stick at winter's first call. Engineers who see Vaillants weekly fix it in their sleep.
Is F75 expensive to repair?
Usually not — sensor and pump are both standard parts with quick diagnosis. The lasting economy is in the follow-up: cleaning up the system water (inhibitor, filter) so the fouling mode doesn't recur.
Can I clear F75 myself?
Check the pressure gauge and top up via the filling loop if low, then one reset — occasionally an air or pressure quirk clears. A returning F75 is component work behind the case: Gas Safe engineer territory.
Why does the code clear and come back?
Intermittent sensor faults — ageing components, wiring connections, moisture — behave exactly that way, as do genuine intermittent system conditions. Note the pattern (immediate return vs. after running) and report it; it halves the diagnostic search.
Can I just replace the sensor myself?
On a gas boiler, no — components behind the case are Gas Safe work legally, and on any boiler the diagnosis should precede the part. A cheap sensor fitted to a system with a real underlying condition just re-fails.
Getting this sorted properly
If reading this has confirmed you need an engineer rather than an afternoon, that's a useful outcome — the expensive version of most heating faults is the one that was almost fixed. The Gas Safe registered engineers in our network do this work daily: they carry the common parts for the mainstream brands, price before starting, and tell you honestly when a repair stops making financial sense on an ageing boiler. Breakdowns get priority response; planned work gets a proper fixed quote after a survey.