Ariston 105 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 105 fault code mean?
Fault code 105 on an Ariston boiler signals that the boiler has detected a dangerously large temperature gap between the flow pipe (water leaving the boiler) and the return pipe (water coming back). Specifically, this difference has exceeded 55°C, which tells the boiler that very little — or no — water is moving around the heating system. If the boiler registers this condition three times consecutively, it will lock out entirely as a safety measure. In simple terms: the boiler is producing heat, but that heat is not being carried away properly because water circulation is severely restricted or absent.
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
- Seized or airlocked circulation pump Common
The circulation pump is responsible for pushing water around the central heating system. If the pump shaft has seized due to wear, scale, or a period of inactivity, or if an airlock has formed inside it, water flow drops sharply and the flow-return temperature difference climbs quickly. This is the single most frequent cause of code 105 on Ariston boilers.
- Sludge or debris blockage in the system Common
Over time, corrosion inside radiators and pipework produces a dark, sludgy deposit that circulates and gradually restricts flow. Severely sludged systems can choke the boiler's internal waterways to the point where the heat exchanger cannot shed heat fast enough, triggering the 55°C differential limit.
- Air trapped in the heating circuit Common
Pockets of air lodged in radiators, the pump housing, or the pipework prevent water from flowing freely. Even a moderate airlock can reduce circulation enough to cause an excessive temperature difference between flow and return, particularly at higher boiler outputs.
- Scaled or partially blocked heat exchanger Sometimes
In hard-water areas, limescale builds up inside the heat exchanger over several years, reducing the water-side flow path and causing localised overheating. The boiler senses the resulting temperature spike and triggers fault 105 even if the rest of the system is relatively clean.
- Faulty flow or return NTC temperature sensor Sometimes
Each NTC thermistor monitors water temperature at a specific point in the circuit. If one sensor drifts out of calibration or develops an intermittent fault, the boiler's control board may calculate a false temperature differential and display code 105 even when actual circulation is perfectly adequate.
- Closed or partially closed isolation valves Rare
If a service valve on the flow or return pipe was left partially closed after previous maintenance, it will throttle the water flow and replicate the symptoms of a pump or blockage fault. Always worth ruling out before assuming a component has failed.
How to fix it
- Do not continue to run the boiler repeatedly DIY safe
Repeatedly resetting and firing a boiler that is showing code 105 risks damaging the heat exchanger through localised overheating. Switch the boiler off at the controls and leave it off while you work through the checks below.
- Reset the boiler once DIY safe
Press and hold the reset button for approximately five seconds as described in your Ariston manual. Allow the boiler to complete its ignition sequence. If the fault clears and does not return within a short test period, monitor it closely over the next few hours. If code 105 reappears, do not reset again — move to the next steps.
- Bleed the radiators to remove trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator bleed key to open each bleed valve slightly until water (not air) trickles out, then close it firmly. Work back towards the boiler. After bleeding, check the system pressure on the boiler's pressure gauge — it should read between 1.0 and 1.5 bar when cold. If it has dropped below 1 bar, top it up via the filling loop.
- Top up system pressure if needed DIY safe
Locate the filling loop (usually a braided hose with one or two valves beneath the boiler). Slowly open the valve(s) and watch the pressure gauge rise. Stop at around 1.2–1.5 bar, then close the valve(s) securely. Leaving the filling loop open can lead to over-pressurisation.
- Check that all radiator and pipework isolation valves are fully open DIY safe
Trace the visible pipework around the boiler and check that every service valve or isolation valve handle is in line with the pipe (open). A valve left at 45° or perpendicular to the pipe is partially or fully closed and will restrict flow significantly.
- Listen and feel for a seized circulation pump Gas Safe engineer
An engineer can remove the pump access screw and manually rotate the pump shaft to check for seizure, or use electrical testing to confirm whether the pump motor is running. A seized pump must be replaced by a Gas Safe registered engineer, as this involves draining down part of the system and working on pressurised pipework.
- Have the NTC temperature sensors tested Gas Safe engineer
A Gas Safe engineer will use a multimeter to compare each sensor's resistance against the manufacturer's specification at a known water temperature. A sensor reading outside tolerance needs replacing — a straightforward job for a professional but not a homeowner task, as it requires draining and accessing internal boiler components.
- Arrange a power flush or magnetic filter installation if sludge is suspected Gas Safe engineer
If the engineer finds evidence of sludge — dark water when bleeding radiators, cold spots on radiators, or visible debris — a power flush using specialist equipment will clear the system. Installing a magnetic filter on the return pipe afterwards catches future particles before they reach the boiler. Both are engineer jobs.
- Have the heat exchanger inspected and descaled if necessary Gas Safe engineer
In hard-water areas or on older boilers, the heat exchanger may need a chemical descale or replacement. Your engineer can assess the severity of scaling and advise whether descaling fluid or a new heat exchanger is the more cost-effective route.
- Call a Gas Safe registered engineer Gas Safe engineer
If the fault persists after bleeding radiators and checking valves, or if you cannot identify a straightforward cause, book a Gas Safe registered engineer. Provide them with the fault code (105) and describe how frequently it has appeared. Ask them to check the pump, sensors, heat exchanger condition, and system cleanliness as a priority.
Parts you may need
- Circulation pump (suitable for Ariston) · from £110
- Flow or return NTC thermistor sensor · from £25
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £65
- Pump head replacement kit · from £55
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix Ariston fault code 105 myself by just resetting the boiler?
A single reset is a reasonable first step and occasionally clears a transient airlock that triggered the fault. However, if the code returns after one reset, continuing to reset the boiler is not advisable — it risks heat exchanger damage and will not address whatever is restricting circulation. Bleeding the radiators and checking system pressure are the only other homeowner-safe actions. Everything else requires a Gas Safe engineer.
How much does it cost to fix an Ariston 105 fault code in the UK?
The total cost depends on the cause. Replacing a circulation pump — the most common culprit — typically costs £150–£380 all-in, covering parts and two to three hours of labour. Replacing an NTC sensor usually comes in at £100–£200. A power flush to clear sludge is often quoted separately and can add £300–£600 depending on system size. In the worst case, if the heat exchanger or PCB needs replacement, costs can rise to £500 or more, but this is much less common with code 105 specifically.
Why does my Ariston boiler show code 105 only in cold weather?
In cold weather the boiler fires at a higher output to meet demand, which makes any existing restriction in circulation far more apparent. A pump that is slightly worn, a system with moderate sludge, or a partially blocked heat exchanger may cope at lower outputs in mild weather but struggle to keep the flow-return differential below 55°C when the boiler is working hard. If code 105 appears mainly in winter, treat it as an early warning — the underlying restriction will only worsen.
Is my Ariston boiler still under warranty if it shows fault 105?
Ariston warranties typically run from two to twelve years depending on the model and whether the boiler was registered at installation. If your boiler is within the warranty period and has been serviced annually by a Gas Safe engineer as required by the warranty terms, the repair may be covered at no cost. Check your warranty documentation or contact Ariston's customer support line with your model number and installation date before booking an independent engineer.