Vaillant F.64 fault codewhat it means and how to fix it
An 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.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.64
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What F.64 means
F.64 sits in the run of electronics codes from F.61 to F.65, and it is the one that overlaps with the sensor family. Vaillant's 2005 and 2009 servicing instructions print it as "electronics/NTC fault — short-circuit in flow or return NTC or electronics defective", and the ecoTEC exclusive, ecoFIT, ecoTEC sustain, open-vent ecoTEC plus and ecoMAX all keep that pair of causes, the ecoFIT and sustain adding "analogue-to-digital converter" to the title because that is the part of the board doing the reading.
The ecoTEC plus ..6/5-5 and pro widen it to "electronics, safety-relevant sensor or cable defective" and have the engineer check the flow sensor, its cable, the flame-recording sensor — the ionisation electrode — for an unstable signal, and then the electronics. The 2022 CS/1-5 gives the most useful instruction of all: "the sensor signal could not be converted correctly — check the sensor displayed under D.166", and replace the PCB only if that diagnostic value is above 30 and unlisted.
In plain terms the board has an input it cannot digitise. When the input is a shorted NTC the repair is the same cheap sensor as F.10 or F.11; when it is the board's converter the repair is the board. The turboMAX plus has the same row as "sensor or electronics fault — check NTCs/connections or electronics defect".
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flow or return sensor short-circuited | Vaillant's first cause on every generation. A shorted NTC drives its input past what the converter can read, and the code is F.64 rather than F.10 or F.11 on some boards. | Common |
| Sensor cable damaged | The ecoTEC plus check list names the cable to the flow sensor. A lead chafed to the casing reads the same as a shorted sensor. | Common |
| Unstable flame signal | The plus and pro tables add the ionisation electrode's signal to the check list, so a corroded or misplaced electrode can present as F.64 on those models. | Occasional |
| Board converter failed | The electronics themselves. Vaillant's D.166 check on the 2022 CS/1-5 exists to separate this from a sensor fault before a board is ordered. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£400
Time on site
1 hour
A sensor is £110 to £170 fitted and a cable repair sits within a call-out. A PCB is £250 to £400 fitted, the expensive end of this range.
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.
Vaillant flow, return or cylinder temperature sensor (NTC)
£12–£38Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant main PCB (electronics box)
£160–£340Authorised 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.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
- F.195Get it checkedA 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.Related: electronics and power
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.64
Does F.64 mean a new board?
Not by default. On every generation Vaillant list a shorted sensor before the electronics, and an engineer measures the sensors first. Boards are replaced when a good sensor still reads wrong.
What is D.166?
A diagnostic code on the 2022 CS/1-5 that displays which sensor the board could not read. An engineer uses it to go straight to the right sensor rather than testing all of them.
Can I reset F.64?
You can press the fault clearance key once. A momentary short from moisture may clear; a failed sensor or converter will not, and the code returns as soon as the board looks again.
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.