Vaillant F.72 fault codewhat it means and how to fix it
The flow and return sensors are reporting a temperature difference that cannot be real. The boiler is blaming the sensors themselves rather than the water.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.72
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 has the boiler been doing?
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.
The code arrived after work was done on the boiler
Transposed or badly refitted sensors are a leading cause, and the fault often does not surface until the appliance runs a full heating cycle.
Return to the engineer who carried out the work before commissioning a fresh diagnosis.
Pressure is normal and the radiators were heating fine before
Consistent with a sensor fault rather than a circulation problem. A boiler with genuinely poor flow usually gives warning through cool radiators first.
Book an engineer to measure both sensors. Do not start adding water or bleeding radiators.
The temperature on the display jumps around oddly
Visible instability in the reported temperature supports a failing sensor or an intermittent connection on one channel.
Mention what you have seen — it helps identify which sensor is at fault.
What F.72 means
F.72 is raised when the temperature spread between the flow and return sensors is invalid — not merely wide, but outside anything the boiler accepts as physically possible. This is the distinction that trips people up. Vaillant has separate codes for a spread that is genuinely too large because water is not circulating; F.72 is different, because the pair of readings does not describe any real state a heating system could be in.
A return warmer than the flow while the burner is running, or two readings that move independently of each other, cannot both be correct. So rather than acting on them, the boiler concludes one or both sensors is lying and stops. The practical consequence is that topping up pressure and bleeding radiators will not help here, because the water is not what is being complained about.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| One sensor drifted out of calibration | An ageing thermistor reads progressively further from true, and eventually the pair no longer agree plausibly. | Common |
| Flow and return sensors transposed | Swapped during a repair, the boiler sees the return apparently warming before the flow, which is impossible. | Common |
| Poor contact at one sensor | A sensor not clipped tight to its pipe lags well behind the other, producing a spread that makes no sense. | Occasional |
| Wiring or connector fault on one channel | Added resistance in one sensor circuit shifts that reading while leaving the other correct. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£280
One sensor £110 to £190 fitted. Both together £160 to £260 as the labour is shared. Wiring repair often within a call-out of £80 to £150.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.0Get it checkedThe flow temperature sensor circuit is broken. The boiler cannot read how hot the water leaving it is, so it has stopped rather than fire blind.Related: temperature sensors
- F.1Get it checkedThe return temperature sensor circuit is open: the boiler is getting no reading from the sensor on the pipe bringing water back in, so it has stopped.Related: temperature sensors
- F.10Get it checkedThe flow temperature sensor has short-circuited. The boiler is reading an impossible value from the sensor on its outlet pipe and will not fire on it.Related: temperature sensors
- F.11Get it checkedThe return temperature sensor has short-circuited. Vaillant treat it as a wiring or sensor fault, and the boiler stays off until the circuit reads correctly again.Related: temperature sensors
- F.84Get it checkedThe flow and return sensors disagree in a way that cannot be true — the return reading warmer than the flow. Vaillant's first explanation is that the two sensors have been swapped.Related: temperature sensors
- F.71Get it checkedThe flow sensor is returning a value the boiler does not believe — typically one that never changes. Vaillant's first suspect is a sensor that has slipped off the pipe.Related: temperature sensors
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.72
How is F.72 different from F.23?
F.23 concerns a spread that is genuinely too large, which points at water not circulating. F.72 concerns a spread that could not occur at all, which points at the sensors. The difference determines whether the repair is to the system or to a component, so it is worth getting right.
Should I top up the pressure?
No, and it is worth resisting the urge. F.72 is not a water shortage code. Adding water introduces fresh oxygenated water into a sealed system for no benefit, and it obscures the real fault.
Will both sensors be replaced?
Sometimes, and it is often sensible. They are inexpensive, they are the same age, and both are reached in the same visit. Paying twice the labour a few months later to change the second one is a false economy.
What should this cost?
One sensor is £110 to £190 fitted, and both together typically £160 to £260 since the labour is shared. A wiring repair may come within a call-out charge.
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.