Vaillant F.195 fault codewhat it means and how to fix it
A 2022 ecoTEC plus code: the boiler has measured a significant undervoltage in its mains supply. F.196 is the overvoltage twin. Vaillant's first instruction is to check the mains, not the boiler.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.195
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.195 means
The ecoTEC plus CS/1-5 measures its own supply voltage — through the pump electronics, as Vaillant's table notes — and stops if it is well outside the range the boiler is designed for. F.195 is "the unit has detected significant undervoltage in the power supply"; F.196 is the same row for overvoltage.
The remedy list is identical for both and unusual for a fault code, because the first item is not in the boiler at all: "check the mains voltage; if the mains voltage is not correct, contact the energy supply company". Only if the mains is correct does Vaillant move to the boiler — "if the mains voltage is correct, replace the pump (voltage measurement via pump electronics)" — and then to the wiring harness between the PCB and the pump.
In other words the pump is the instrument, and a pump whose electronics misreport the supply gives a false F.195 on a healthy mains. Undervoltage from the network is not rare in Britain: a long rural supply under load, a shared supply with a heavy neighbour, or a fault at the substation can all pull the voltage low enough for the boiler to notice before anything else in the house does. The row has no two-digit ancestor; earlier ecoTECs simply stopped or reported an electronics fault when the supply misbehaved.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Mains supply voltage low or high | Vaillant's first cause and first instruction. A network problem the energy supplier has to fix; the boiler is reporting it, not causing it. | Common |
| Pump electronics misreading the supply | The boiler measures voltage through the pump. A pump whose electronics have drifted reports a fault that a meter on the mains does not confirm, and Vaillant's remedy is then the pump. | Occasional |
| Wiring harness between PCB and pump | Vaillant's third item. A poor connection on the pump lead drops the voltage the pump sees, and the board reads it as an undervoltage. | Occasional |
| Poor connection in the house wiring | Not on Vaillant's list but between their first two: a loose terminal at the boiler's fused spur or a worn isolator can drop the voltage at the boiler alone. An electrician's check if the supplier finds nothing. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£320
Time on site
45 minutes
A network voltage problem costs nothing to report and is the supplier's to fix. A call-out to check the boiler is £80 to £120; a pump, if it is the instrument at fault, is £250 to £320 fitted.
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.63Get it checkedVaillant reports an EEPROM error, meaning the boiler cannot reliably read its own stored settings. The printed circuit board is at fault and needs replacing.Related: electronics and power
- F.70Get it checkedThe boiler does not know which model it is. The device specific number is missing or wrong, which nearly always follows a board or display replacement.Related: electronics and power
- F.64Get it checkedAn electronics or sensor fault: the board cannot make sense of a safety-relevant sensor signal. Vaillant's causes are a shorted flow or return sensor or a defective board.Related: electronics and power
- F.65Get it checkedThe electronics have got too hot. Vaillant say either something outside the boiler is heating the board, or the board itself is faulty; the boiler stops until it cools.Related: electronics and power
- F.67Get it checkedThe flame signal the board is receiving is not believable. Vaillant treat F.67 as an electronics fault in the flame-monitoring circuit rather than a real flame problem.Related: electronics and power
- F.97Get it checkedVaillant's main PCB has failed the check it runs on itself. The table gives one cause, "main PCB defective", and one remedy: replace the PCB.Related: electronics and power
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.195
Who do I call — a heating engineer or the electricity company?
Vaillant say check the mains first. If lights dim or flicker, or the code arrives when something heavy switches on, ring your network operator on 105 and report a voltage problem. If the supply proves sound, a Gas Safe engineer checks the pump and its harness.
Does F.196 mean the same thing?
It is the overvoltage version, with Vaillant's remedies in the same order: mains first, then the pump, then the harness. This page covers both.
Will the boiler restart on its own?
If the supply recovers it may clear with a reset — a press of longer than three seconds on the CS/1-5. If it keeps returning at the same time of day, the supply is the pattern to report.
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.