Ariston 604 fault codewhat it means and how to fix it
Fan speed is outside the range the appliance expects, so it will not allow the burner to light. This one needs an engineer.
Take care
Get a Gas Safe registered engineer to look at your Ariston 604
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 604 means
The leading 6 marks this as the air inlet and flue gas outlet group, and Ariston's description of 604 is "insufficient fan speed". The appliance commands a speed, reads the feedback from the fan, and refuses to open the gas valve unless the two agree — the fan is what removes combustion products through the flue, and firing without proven extraction is not something any manufacturer permits. Which number you see depends on the generation, and it is worth being clear about because the lists on the open web are not.
604 is the Clas HE Evo's code. On the current One series the same fault is 612, described as insufficient fan speed on the Clas One and as a fan error with the speed higher or lower than the set values on the Genus One+. The renumbering matters more than it sounds, because 610 moved too: on the Evo it is a thermofuse, on the One it is an overheat of the main heat exchanger.
Two things realistically cause the fan fault. The first is a worn fan: bearings dry out after eight to twelve years and the fan slows or stops, usually after months of grumbling on start-up that the household stops registering. The second is a flue obstruction, which alters the resistance the fan works against and throws the speed reading even though the fan is fine. Checking the flue before condemning the fan is the difference between a call-out charge and a four-hundred-pound part.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Fan bearings worn out | The leading cause on appliances past eight years or so. Listen back for whether there has been increasing noise on start-up over recent months. | Common |
| Flue terminal obstructed | Bird nest, leaf debris, or render pushed into the terminal during building work. Enough to change fan loading and fail the check. | Occasional |
| Fan wiring or connector fault | The fan spins but the tachometer signal never arrives at the board. Inexpensive if that is all it is. | Occasional |
| Control board fault | The board misreads a healthy signal. Only concluded after the fan and flue have been properly eliminated. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£180–£420
A replacement fan is typically £280 to £420 fitted. If it turns out to be a flue obstruction or a connector, expect a call-out charge of £80 to £140.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
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Related Ariston fault codes
- 501Get it checkedNo flame was detected during ignition and the appliance has locked out. Check the gas supply and the condensate pipe before booking anyone.Related: same brand
- 108Usually simple to fixThe heating circuit does not contain enough water to run safely. Refill through the filling loop to about 1.2 bar and the code clears.Related: same brand
- 101Get it checkedThe heating circuit has overheated and the appliance has shut down. The usual reason is that water has stopped moving, not that it is firing too hard.Related: same brand
- 103Get it checkedThe appliance is reporting insufficient circulation — water is not moving through it fast enough. Worth acting on before it becomes an overheat.Related: same brand
- 303Get it checkedThe main control board has reported an internal fault. It needs an engineer, and it is worth checking the electrical supply before assuming the board is dead.Related: same brand
- 109Get it checkedSystem pressure has gone above 3 bar. Usually this means somebody overfilled it, or the expansion vessel has failed.Related: same brand
Own an Ariston boiler? Save it and its codes come first next time.
Related guides
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.