Ariston 604 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 604 fault code mean?
Fault code 604 on Ariston boilers indicates that the fan is not reaching the speed required for safe operation, or that the Hall sensor responsible for monitoring fan speed has reported a problem. The boiler's control system continuously checks that the fan is spinning at the correct rate before and during a heating or hot water cycle — if it falls short, the boiler locks out rather than risk combustion gases failing to vent properly. This is a safety shutdown, not a nuisance trip, so it should be taken seriously and investigated promptly.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Worn or failing fan motor Common
Over time the motor bearings wear and the windings can deteriorate, meaning the fan can no longer reach the speed the boiler expects. This is the single most common reason for a persistent 604 code, particularly on boilers that have not had a recent service.
- Dirty or obstructed fan impeller Common
A build-up of dust, fluff, or debris on the fan blades increases rotational resistance and slows the fan below its target speed. Boilers in dusty or poorly ventilated locations are especially prone to this.
- Faulty Hall sensor Sometimes
The Hall sensor is a small electronic component that reads the fan's rotational speed and sends the signal back to the PCB. If the sensor itself has drifted out of calibration or failed, the boiler will log a 604 fault even if the fan is mechanically sound.
- Blocked or partially blocked flue Sometimes
Significant resistance in the flue — caused by a bird's nest, ice, or a collapsed terminal — can overload the fan motor and prevent it from reaching the required speed under normal power.
- Wiring or PCB fault Rare
Damaged wiring between the fan, Hall sensor, and PCB, or a fault on the PCB itself, can cause incorrect speed readings or prevent the fan from receiving a proper voltage supply.
How to fix it
- Listen and observe when the boiler tries to fire DIY safe
Before attempting anything else, press the reset button (hold for approximately 5 seconds until the boiler responds) and listen carefully. You should hear the fan spin up before ignition attempts. A grinding or sluggish sound from the fan area — or complete silence — tells an engineer exactly what to investigate first. Limit resets to two or three attempts; repeatedly resetting will not fix the underlying problem.
- Check the flue terminal externally DIY safe
Go outside and visually inspect the flue outlet (the white or grey pipe on an exterior wall or roof). Ensure there is nothing blocking it — frost, a bird's nest, or debris. Do not reach inside the flue; a visual check only. If you find an obvious blockage you cannot safely clear from ground level, note this for the engineer.
- Confirm the boiler is due a service DIY safe
Check when the boiler was last serviced. Ariston recommend annual servicing, and a fan fault is frequently the first sign that internal components have not been cleaned in several years. Booking a service alongside the diagnosis call can save a second call-out charge.
- Contact a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
All further investigation — removing the fan, testing motor voltage and speed, inspecting the Hall sensor, checking wiring continuity, and examining the PCB — must be carried out by a Gas Safe registered engineer. The engineer will use diagnostic equipment to confirm whether the fan, sensor, wiring, or PCB is at fault before ordering parts. Do not attempt to open the boiler casing or interfere with any internal components yourself.
- Engineer to replace the fan and/or Hall sensor if required Gas Safe engineer
If testing confirms the fan motor is worn or the Hall sensor has failed, the engineer will source a compatible replacement part for your Ariston model and fit it. Once replaced, the boiler should be run through a full heating cycle to confirm the fault code has cleared and the fan speed readings are within the manufacturer's specification.
Parts you may need
- Ariston fan assembly (model-specific) · from £120
- Hall sensor / fan speed sensor · from £25
- Fan wiring loom · from £30
- PCB (if wiring checks implicate the board) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £220–£380, depending on the underlying cause.
Frequently asked questions
Is fault code 604 dangerous — should I leave the boiler off?
The boiler has already locked itself out as a precaution, which is the correct response. You should leave it in lockout and not attempt to force it to run by repeatedly resetting it. Without a correctly functioning fan, combustion gases may not vent safely. Leave the boiler off until a Gas Safe engineer has inspected it.
Can I reset the boiler myself and will that fix the 604 code?
You can attempt one or two resets — hold the reset button for around 5 seconds. Occasionally the code is triggered by a momentary sensor glitch and a single reset clears it. However, if the fault returns after resetting, the underlying component (fan motor, Hall sensor, or wiring) needs proper diagnosis. Repeated resetting without a fix is not safe and will not resolve a mechanical or electrical fault.
How much does it cost to fix an Ariston 604 fault code?
The most common repair — fan replacement including parts and labour — typically costs between £220 and £380 in the UK (2025 prices). A diagnosis-only call-out usually runs £100–£150, which is often credited against the repair if you proceed. If the fault turns out to be a faulty Hall sensor or a wiring issue rather than the fan itself, the cost is usually at the lower end. In rare cases where the PCB is implicated, costs can rise above £400; if your boiler is older and facing multiple component failures, it is worth asking the engineer for a replacement boiler quote alongside the repair quote.
Which Ariston boiler models are most affected by code 604?
The 604 fault code most commonly appears on the Ariston CLAS HE R, E-Combi Evo, E-System Evo, Clas HE Evo Combi, and Clas HE Evo System. The fan assembly and Hall sensor design are broadly similar across these ranges, so diagnosis and repair procedures are consistent. Always quote your full model name and serial number when ordering replacement parts, as fan specifications can vary between production runs.
Could a blocked flue cause the 604 fault code?
Yes, though it is less common than a worn fan. If the flue outlet is heavily restricted — for example by a frozen condensate pipe in the same area, ice at the terminal, or a bird's nest — the fan has to work much harder and may fail to hit its speed target. Check the flue terminal externally for obvious blockages. If the flue is clear, the fault almost certainly lies within the boiler itself.