Vaillant F.94 fault codewhat it means and how to fix it
The vortex and differential pressure sensors — the two that measure flow through the heat exchanger on an ecoTEC plus or exclusive — are returning values the board does not believe.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.94
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.94 means
The ecoTEC plus ..6/5-5 and exclusive measure water flow through the primary circuit with a vortex sensor and read the pressure drop across it with a differential pressure sensor. Together they let the board judge circulation directly rather than infer it from temperatures. F.94 is those readings failing plausibility: the plus prints "fault: vortex and differential pressure — sensors return implausible values; check cable harness, plug, sensors", and the exclusive adds the pump to the front of the list, "check: pump (function), cable harness, plug, sensors". The exclusive's addition is the useful clue.
If the pump is not moving water, both sensors report something the board considers impossible, so a pump fault can present as F.94 before it presents as anything else. The remaining causes are the sensors' wiring and the sensors themselves. F.94 is not on the pro, ecoFIT, ecoTEC sustain or 2022 CS/1-5 tables — the pro and ecoFIT judge circulation from the temperature spread and report it as F.23, F.24 or F.72, and the CS/1-5 has a family of F.32x pump codes for the same territory. It is an engineer's repair throughout.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump not circulating | First on the exclusive's list. A stalled or failing pump leaves no flow for the vortex sensor to read and no pressure drop for the differential sensor. | Common |
| Sensor plug or harness fault | Vaillant's cable harness and plug. Two sensors, two leads, and a disturbed connector on either gives the code. | Common |
| Vortex or differential pressure sensor failed | The sensor itself. Diagnosed by comparing its output with the other and with a known flow. | Occasional |
| Air in the primary circuit | Air passing the vortex sensor produces readings that jump about. Not on Vaillant's list by name, but a reason the pump check comes first. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£320
Time on site
1 hour
A plug or harness fault is a call-out of £80 to £150. A sensor is £120 to £200 fitted; a pump £250 to £320.
Typical UK prices including parts and labour, updated September 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.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
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Related Vaillant fault codes
- F.325Get it checkedThe heating pump in a 2022 ecoTEC plus has reported a fault — blocked, or defective. F.320, F.321, F.324 and F.330 are its neighbours in the same pump family, and this page covers them too.Related: pump and hydraulics
- F.89Get it checkedThe pump is not connected, is connected incorrectly, or is not the pump the boiler expects. On the 2022 CS/1-5, F.089 means the fitted pump does not match the unit type.Related: pump and hydraulics
- F.323Get it checkedThe heating pump in a 2022 ecoTEC plus is dry running — turning with no water in it. Vaillant name air in the boiler as the cause; a pump run dry may need replacing.Related: pump and hydraulics
- F.322Get it checkedThe heating pump in a 2022 ecoTEC plus has overheated, and running it more gently did not bring the temperature down. Vaillant's remedy is to check and if necessary replace the pump.Related: pump and hydraulics
- 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: same brand
- 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: same brand
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
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
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.