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
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What 604 means
The leading 6 marks this as the air inlet and flue gas group. The appliance commands a fan speed, reads the feedback from the fan, and refuses to open the gas valve unless the two agree — because the fan is what removes combustion products through the flue, and firing without proven extraction is not something any manufacturer will permit. Ariston also list 612 on some models as a fan error where the speed is higher or lower than the set values, which is the same interlock described slightly differently. Two things realistically cause it. 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 August 2026. Your quote may differ.
What other owners found
Nobody has shared what worked for them on this code yet. If you have had it, you would be the first — and it genuinely helps the next person.
Related Ariston fault codes
- C6 224Get it checkedThe fan is not reaching the speed the boiler asked for, so combustion cannot be confirmed as safe and the burner has been prevented from firing.Related: fan and flue
- E16Get it checkedThe control board cannot confirm the fan is running at the speed it asked for, so it will not let the burner light. This one needs an engineer.Related: fan and flue
- F.32Get it checkedThe fan is not running at the speed the boiler expects, so combustion cannot be verified as safe and the boiler has locked out.Related: fan and flue
- E160Get it checkedThe fan is not running at the speed the appliance expects. Baxi direct engineers to check the fan and its cable, in that order.Related: fan and flue
- F3Get it checkedThe fan is not running at the speed the boiler expects, so it cannot confirm the flue is clearing and has stopped the burner firing.Related: fan and flue
- F9Get it checkedThe fan is not reaching the speed the burner control unit requires. The burner is blocked until it does.Related: fan and flue
Related guides
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
- Interviews with working engineers
Checked by Technical reviewer (Gas Safe registration to be displayed on appointment) on 5 August 2026.