Ariston 104 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 104 fault code mean?
Ariston fault code 104 is a safety lockout triggered when the boiler detects that water is not circulating properly around the heating system. More specifically, the boiler's sensors register that the temperature difference between the flow and return pipework is climbing faster than it should — the threshold is typically a rise of more than 20°C per second. When water is moving freely, heat is carried away from the heat exchanger at a steady rate; if circulation is poor or absent, heat builds up rapidly and the boiler shuts down to protect itself. On some older Ariston models (including certain Genus variants), code 104 may also be linked to insufficient system water pressure rather than a flow-rate issue, and the boiler will not reset automatically until the underlying problem is corrected. Either way, the root cause is the same: water is not travelling around the circuit as the boiler expects.
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
- Blocked or seized circulation pump Common
The circulation pump is responsible for pushing water around the heating circuit. Over time — especially in boilers that have sat idle for a long period — the pump impeller can seize due to limescale, sludge, or simple corrosion. A completely seized pump means no water movement at all, which triggers the 104 lockout almost immediately after the boiler fires.
- Sludge and debris build-up in the system Common
Black iron oxide sludge accumulates in heating systems over years of use as radiators and pipework corrode internally. This debris can restrict flow through radiators, pipework, and the boiler's internal waterways. Even a partial blockage can reduce circulation enough to cause the rapid temperature rise that triggers code 104.
- Low system water pressure Common
If the system pressure has dropped below around 0.8–1 bar, there may not be enough water in the circuit for the pump to push around effectively. Some Ariston models will display a 'Refill system' message alongside code 104 when this is the cause. The correct operating pressure is typically 1–1.5 bar when the system is cold.
- Pump airlock Sometimes
Air trapped inside the pump body prevents the impeller from moving water efficiently. This can happen after a system drain-down, following maintenance, or when air gradually accumulates in a system that has not been properly vented. The pump may be running — you can often hear it — but pushing little or no water.
- Faulty or obstructed flow sensor Sometimes
The flow sensor (sometimes called a flow switch or flow rate sensor) monitors water movement through the boiler. If this component is coated in debris, physically damaged, or has developed an electrical fault, it may report zero or inadequate flow even when the pump is working normally. This leads the boiler's PCB to trigger code 104 as a precaution.
- Stuck or faulty diverter (three-way) valve Sometimes
Combination boilers use a diverter valve to switch water between the central heating circuit and the domestic hot water heat exchanger. If this valve sticks in an intermediate position or seizes entirely, water cannot circulate freely through the heating circuit, causing the boiler to detect poor flow and lock out.
- Worn pump bearings or broken shaft Rare
Even if the pump is not completely seized, worn internal bearings or a damaged shaft can reduce its output dramatically. The motor may run but the impeller turns too slowly to push adequate water volume around the circuit. This tends to be a gradual deterioration rather than a sudden failure.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button on the control panel for approximately 5 seconds until the boiler attempts to restart. A single reset is worthwhile — a transient airlock or momentary sensor glitch can sometimes clear itself. However, if code 104 returns straight away or after a short run, do not keep resetting repeatedly, as this will not fix an underlying mechanical problem and can mask a fault that needs proper diagnosis.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, the circuit is under-filled. Locate the filling loop (usually a braided flexible hose with one or two valves beneath the boiler), open the valve(s) slowly, and watch the gauge rise. Stop when the needle reaches approximately 1.2–1.3 bar, then close the filling loop valves securely. If the pressure keeps dropping over days or weeks, there is a leak somewhere that needs a Gas Safe engineer to find and repair.
- Inspect for visible leaks DIY safe
Walk around the property checking radiator valves, visible pipework, and the area beneath the boiler for drips, damp patches, or dried white mineral deposits (a sign of a previous slow leak). Even a small weeping joint can cause the system to lose pressure over time. Do not attempt to repair any leak yourself — report the location to a Gas Safe engineer.
- Bleed the radiators to remove trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator bleed key to open the bleed valve (small square nipple at the top of the radiator) until water trickles out steadily without sputtering. Work your way back towards the boiler. After bleeding all radiators, re-check the system pressure — bleeding releases air but also drops the pressure slightly — and top up via the filling loop if needed.
- Call a Gas Safe registered engineer to inspect the circulation pump and internal components Gas Safe engineer
If the fault persists after the homeowner steps above, the most likely cause is a seized, airlocked, or failing circulation pump, a blocked flow sensor, or a stuck diverter valve. A Gas Safe engineer can physically inspect the pump, attempt to free a seized impeller, test the flow sensor electrically, and assess whether the pump needs cleaning or full replacement. They can also arrange a power flush if significant sludge is found in the system. Do not attempt to open the boiler casing, tamper with the pump, or touch any gas components yourself.
Parts you may need
- Circulation pump (compatible replacement) · from £80
- Diverter / three-way valve · from £55
- Flow sensor / flow switch · from £30
- Magnetic system filter (e.g. Fernox TF1 or Magnaclean) · from £65
- Inhibitor fluid (post-flush treatment) · from £18
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 £150–£400, depending on the underlying cause.
Frequently asked questions
Can I reset an Ariston 104 fault myself, or do I need an engineer?
You can safely attempt one reset, re-pressurise the system to 1–1.5 bar, bleed the radiators, and check for obvious leaks. These steps fix the fault in a minority of cases — particularly when low pressure or trapped air is the culprit. However, if code 104 keeps coming back, the cause is almost certainly mechanical (a failing pump, sludge blockage, or faulty flow sensor), and a Gas Safe registered engineer will need to investigate. Repeatedly resetting without fixing the root cause risks damaging the heat exchanger.
How much does it typically cost to fix Ariston fault code 104 in the UK?
For most households, the repair falls between £150 and £400 including parts, labour, and VAT. A straightforward pump clean or airlock clearance is at the lower end; a full pump replacement typically costs £150–£380 depending on the model and your location. If the system is heavily silted, a power flush adds £300–£500 on top. In rare cases where the PCB or heat exchanger is also damaged, costs can climb to £500 or more — but that is unusual for a code 104 fault on its own. Prices in London and the South East tend to run around 15–25% higher than the national average.
Why does my Ariston keep showing fault 104 even after I top up the pressure?
If re-pressurising clears the fault briefly but it returns, the boiler is likely losing pressure through a leak, or the root cause is not pressure at all but a circulation problem — most commonly a failing pump or heavy sludge in the system. A Gas Safe engineer can pressure-test the system to locate a leak, or run diagnostics on the pump and flow sensor to identify what is actually restricting water movement.
Is it worth getting a power flush if I have recurrent Ariston 104 faults?
If an engineer confirms that sludge is causing restricted circulation, a power flush is often the most effective long-term solution. It removes the corrosion deposits that are starving the pump and blocking waterways, and when combined with a magnetic filter and fresh inhibitor, it can extend the life of your boiler significantly. The typical cost is £300–£500 for an average-sized home. It is not always necessary — if the pump has simply seized and the system is relatively clean, pump replacement alone may suffice — so get a professional assessment first.