- Boilers
- Gas boiler
F00
Vaillant F.00 on an ecoTEC boiler: flow sensor interruption
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Vaillant F.00 means the flow temperature sensor is disconnected or open circuit. It is not something you can fix: note the code, do not open the boiler and call the official service centre to check the sensor and its connection.
Safety first
Stop and check this first
- Do not open the boiler to check the sensor: Vaillant reserves repairs for qualified professionals and there is mains voltage and hot parts inside.
- If you smell gas: do not operate switches, plugs, doorbells or phones, do not light flames or smoke, open doors and windows, close the meter valve or the gas shut-off valve if you can, leave the building and call the gas company's emergency service from outside. This is what Vaillant asks.
Safe checks you can perform
- Code on the display and what still works
- Recent work on the boiler
- Sensor, connector and harness check by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Harness and connector continuity
- Do not attempt this professional task: Flow sensor measurement
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Harness and connector continuity
What they doThe technician checks the sensor connector and the harness continuity to the board.
They must show youthe connector or section checked.
In writingthe repair done and, where relevant, why the board was not replaced.
Flow sensor measurement
What they doThe technician measures the flow temperature sensor with a multimeter and compares the value with Vaillant's specification.
They must show youthe measured value against the specification.
In writingthe value and whether the sensor was replaced.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Code on the display and what still works
- Who:
- you.
- How:
- take a photo of the code and check whether the boiler gives hot water or heating.
- Result that matters:
- whether the boiler stays locked with F.00.
- If F.00 stays on screen:
- call the official service centre, as Vaillant says for faults you cannot fix yourself.
- If F.00 comes and goes:
- note when; an intermittent contact points to the connector or cable.
- Evidence to keep:
- photos of the code with date and time.
Evidence: Lista oficial de códigos de error de calderas Vaillant · Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Recent work on the boiler
- Who:
- you.
- How:
- think whether anyone opened the boiler recently (service, repair).
- Result that matters:
- whether F.00 started right after it.
- If it started after some work:
- tell the technician; it points to a loose connector.
- If there was no work:
- the sensor or cable may have failed on its own.
- Evidence to keep:
- date of the last service and the job sheet.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Sensor, connector and harness check by the technician
- Who:
- official service centre or authorised technician.
- How:
- checks the flow sensor, connector, harness and board, in that order.
- Result that matters:
- where the break is.
- If the connector was loose or the cable cut:
- that confirms the cause.
- If connector and cable are fine and the sensor does not respond:
- the sensor is replaced.
- Evidence to keep:
- a report of what was measured and replaced.
Vaillant reserves inspection, maintenance and repair for qualified professionals; opening the boiler is not the user's job.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Vaillant
- Product type
- Gas boiler
Known code variants
- ecoTEC F.00
- Error F00 Saunier Duval
- Error F00 Vaillant
- F.0
- F.00
- Saunier Duval F.00
- ThemaFast Condens F.00
- Vaillant F.00
What it means
In the ecoTEC plus fault code table, F.00 is a flow sensor interruption: the flow temperature sensor is faulty or not connected. Vaillant says to check the sensor, the connector, the cable harness and the printed circuit board. If the sensor is short circuited the code is a different one, F.10. The board can no longer see the flow temperature and cannot control the burner; the code does not say which of those parts has failed.
Warnings and stop conditions
- Mediumsource verified
Do not open the boiler to check the sensor: Vaillant reserves repairs for qualified professionals and there is mains voltage and hot parts inside.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Criticalsource verified
If you smell gas: do not operate switches, plugs, doorbells or phones, do not light flames or smoke, open doors and windows, close the meter valve or the gas shut-off valve if you can, leave the building and call the gas company's emergency service from outside. This is what Vaillant asks.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Symptoms and causes
Probable causes · in order
- Sensor connector loose or disconnectedPossible
Vaillant lists the sensor not connected and the connector. It fits if F.00 appears after a recent repair or service.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Broken cable harnessPossible
Vaillant says to check the harness. A cut cable gives the same code as a broken sensor; a continuity test separates them.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Faulty flow sensorPossible
If the connector and cables are fine, the sensor may be open circuit. Measuring it confirms this.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Printed circuit boardLess common
Vaillant lists it last. It is only considered if sensor, connector and harness are fine.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Virtual technician
Does F00 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Lista oficial de códigos de error de calderas Vaillant
Vaillant Group · 2026 · official_doc
- 2
Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Vaillant · 2017 · manual
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