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Vaillant Boiler F.00 Error Code
Flow or return temperature sensor not reading on Vaillant boilers.
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The boiler has lost the signal from one of its water temperature sensors (NTC thermistors). F.00 is the flow sensor, F.01 the return. A loose plug, a damaged wire or a failed sensor is behind it.
Compatible models
The ecoTEC plus VU ..6/6-5 OVZ manual lists F.00 and F.01. Vaillant doesn't say which other models use them, so check your manual.
Also shown as
F.01 is the same fault on the return sensor. A short circuit on the same sensors shows as F.10 or F.11.
Difficulty
Try the basic checks, but the usual fix is a technician's job.
Not sure you are reading the code right? See how to read Vaillant boiler codes.
How to fix Vaillant F.00
Restart once
Switch the boiler off for a minute, back on, and see if the code clears. If it returns, leave it off and arrange a visit.
When to call a technician
The sensors sit inside the casing, so a Gas Safe engineer has to check the connector and replace the sensor if needed.
Parts linked to F.00
These are the parts an engineer usually checks for this code. They sit inside the sealed casing, so leave them to a Gas Safe registered engineer.
Frequently asked questions
Which Vaillant models show the F.00 code?
The ecoTEC plus VU ..6/6-5 OVZ manual lists F.00 and F.01. Vaillant doesn't say which other models use them, so check your manual.
Can I fix Vaillant F.00 myself?
Only the outside checks. The steps above are the ones Vaillant's user instructions allow; if they don't clear F.00, the fix is inside the boiler and needs a Gas Safe registered engineer (RGII in Ireland).
Other Vaillant boiler codes
- F.22Water pressure in the heating system is too low
- F.28Boiler failed to light
- F.29Flame went out and the boiler couldn't relight
- F.20Overheat safety cut-out has tripped
- F.23Too big a gap between flow and return temperature
- F.24Temperature rising too fast
- F.25Flue gas temperature too high
- F.27Flame detected with the burner off
- F.75No pressure rise when the pump starts
- F.10Temperature sensor short circuit
- F.13Hot water cylinder sensor short circuit
- F.32Fan speed out of range
- F.61Gas valve control fault
- F.62Flame took too long to go out
- F.68Unstable flame
- F.70Invalid device code after a repair
- F.83Dry fire: no temperature change when the burner lit
Last reviewed 2026-10-21. Codes and steps can vary by model; check your owner's manual.