Vaillant F.83 fault codewhat it means and how to fix it
The burner lit but neither sensor saw the temperature rise it should have. Either water is not moving through the boiler, or a sensor is misplaced.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.83
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.
Has the boiler been worked on recently?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Yes, within the last few weeks
Start there. Sensor placement is the leading cause of F.83 and it is a mistake that only shows itself once the boiler runs a full cycle.
Go back to whoever carried out the work. Refitting a sensor correctly is a few minutes and should not be a chargeable visit.
No, and the pressure gauge reads under 1.0 bar cold
Air rather than sensor placement. There is not enough water in the appliance for the sensors to see the burner doing anything.
Bring the pressure to between 1.0 and 1.5 bar cold, bleed the radiators, then reset once.
No, and you cannot hear the pump running
A pump that is not turning at ignition produces exactly this. Put your hand on the pump body — a working pump hums and feels faintly alive.
Book an engineer to test the pump and its supply.
What F.83 means
F.83 is raised when the burner starts and the flow and return sensors register little or no increase in temperature. In normal operation the moment the burner lights, the water passing the flow sensor begins warming within seconds. If that does not happen, one of two things is true. Either there is not enough water inside the appliance for the sensors to detect anything happening, or the sensors are not actually reading the water they are supposed to be reading.
The second explanation is more common than people expect, and it is the reason F.83 has a reputation as a post-repair code: a sensor that has been removed and refitted onto the wrong pipe, or left sitting proud of it rather than clipped tight against the metal, cannot see the temperature change even though the boiler is working perfectly.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor refitted in the wrong position | A flow or return sensor clipped onto the wrong pipe after a repair reads a temperature that never changes as expected. | Common |
| Sensor not making proper contact with the pipe | A clip that is not fully home leaves an air gap, and the sensor then responds far too slowly to register the rise. | Common |
| Not enough water inside the appliance | An air pocket in the primary circuit means the burner is heating very little water, so the sensors see almost nothing. | Common |
| Pump not circulating at start-up | If the pump fails to run when the burner lights, heat stays put and no rise appears at either measuring point. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0–£700
Nothing where a sensor was misplaced during recent work under warranty. Sensor £110 to £190 fitted. Pump £180 to £320. Power flush £400 to £700 where sludge is the underlying cause.
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
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.20Stop — call an engineerThe overheat thermostat has tripped because the boiler got too hot. It has shut down to protect itself and it should not be restarted repeatedly.Related: overheat and circulation
- F.23Get it checkedVaillant call this a safety shutdown for temperature spread too great. It nearly always means the pump is not moving enough water.Related: overheat and circulation
- F.24Get it checkedTemperature is rising faster than it should when the burner fires. That is a circulation problem caught a step earlier than an overheat.Related: overheat and circulation
- F.25Get it checkedOn a current ecoTEC this is a flue gas temperature safety shutdown. The same number meant a cable break on the 2009 ecoTEC and a water switch fault on the ecoMAX.Related: overheat and circulation
- F.76Get it checkedThe overheating protection built into the primary heat exchanger has responded. A thermal fuse on the heat cell, or its wiring, has opened, and the boiler has stopped.Related: overheat and circulation
- 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
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.83
How is F.83 different from F.24?
They are opposite readings of the same measurement. F.24 fires when the temperature climbs too quickly, which means very little water is moving. F.83 fires when it barely climbs at all. Both concern circulation, but F.83 additionally raises the possibility that the sensors are simply in the wrong place.
Why would a sensor be in the wrong place?
Because flow and return sensors are physically similar and sit close together on adjacent pipes. During a repair they are often unclipped to get at something else, and putting one back on the wrong pipe is an easy mistake. The boiler cannot detect the swap directly — it just sees temperatures that make no sense.
Can I fix this myself?
You can check the pressure gauge and bleed radiators, which resolves the air-related cases. Anything involving the sensors is inside the sealed casing of a gas appliance and is work for a Gas Safe registered engineer.
What is the likely bill?
If it is sensor placement following recent work, it should cost nothing. Otherwise a sensor is £110 to £190 fitted, and a pump £180 to £320. Add a flush at £400 to £700 only if the system water turns out to be badly contaminated.
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 support portal
- 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.