Ariston 106 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 106 fault code mean?
Fault code 106 on Ariston boilers signals an insufficient circulation problem. The boiler's control system has detected that the return pipe temperature is reading at least 10°C higher than the flow (supply) temperature, and this has happened on three consecutive occasions. Physically, the return water cannot be hotter than the water leaving the boiler under normal operation, so when this reading appears repeatedly it tells the boiler something is seriously wrong — either water is barely moving around the system, or a sensor is giving a false reading. As a safety measure, the boiler locks out to prevent overheating and potential damage. Do not repeatedly reset and continue running the boiler; the underlying cause needs to be identified and resolved.
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
- Faulty or seized circulation pump Common
The circulation pump is responsible for pushing heated water around the central heating system and back through the boiler. If it has seized, is running too slowly, or has failed entirely, water flow drops dramatically. With little or no movement through the system, the temperature readings between flow and return become erratic, triggering code 106. A seized pump is one of the most frequent causes of this fault.
- Sludge or debris blockage Common
Over time, corrosion inside radiators and pipework produces iron oxide sludge that circulates through the system. Heavy sludge deposits can partially or fully block the heat exchanger or pipework, severely restricting water flow. Reduced flow causes uneven temperature distribution, which can result in the return appearing hotter than the flow to the boiler's sensors.
- Airlocks in the system Common
Air trapped in the circulation pump or pipework acts as a barrier to water movement. Even a moderate airlock can reduce flow enough to cause the temperature imbalance that triggers fault 106. Automatic air vents that have become clogged are a common contributor, as they can no longer release trapped air naturally.
- Faulty NTC return temperature sensor Sometimes
If the sensor measuring the return water temperature is giving inaccurate readings — for example, reading far higher than the actual water temperature — the boiler may log a 10°C differential that does not actually exist. A damaged, corroded, or drifting NTC thermistor can produce exactly this kind of false fault trigger.
- PCB (printed circuit board) fault Sometimes
The main PCB processes all sensor inputs and controls boiler operation. If the board is damaged — due to moisture ingress, a power surge, or component failure — it may misinterpret correct sensor data or fail to drive the pump properly, leading to a spurious or genuine circulation fault. Signs of PCB trouble include visible burn marks, the fault returning immediately after other causes have been ruled out, or the boiler not responding normally to commands.
- Electrical wiring or polarity issue Rare
Incorrect polarity at the boiler's electrical connection, or loose/corroded wiring within the system, can cause erratic sensor readings and control issues that manifest as fault 106. This is less common but worth checking, particularly on recently installed or moved appliances.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button for approximately 5 seconds and allow the boiler to restart. A single reset is reasonable — the fault may have been a one-off anomaly. If code 106 returns promptly, do not keep resetting; repeated resets without fixing the root cause can mask a developing problem and risk damaging the heat exchanger.
- Bleed your radiators to remove trapped air DIY safe
Turn the heating off and allow it to cool slightly. Using a radiator bleed key, open the bleed valve on each radiator (starting upstairs and working down) until water flows steadily with no sputtering. Close each valve once water appears. After bleeding, check the boiler's pressure gauge — if it has dropped below 1 bar, top up via the filling loop until it reads 1–1.5 bar. This addresses airlocks that could be restricting circulation.
- Check system pressure is correct DIY safe
Look at the pressure gauge on the boiler — it should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, use the filling loop (usually a braided hose with two valves under or near the boiler) to slowly top it up. Low pressure alone can impair circulation. Do not pressurise above 2 bar.
- Have a Gas Safe engineer inspect and test the circulation pump Gas Safe engineer
An engineer will check whether the pump is running, measure its speed and output, and determine whether it has seized or is underperforming. In many cases the pump can be freed if recently seized, but a failed pump will need replacing. Pump replacement on an Ariston boiler typically takes one to two hours including parts.
- Arrange a power flush if sludge is suspected Gas Safe engineer
If the engineer finds evidence of heavy sludge — such as cold spots on radiators, discoloured system water, or restricted flow — a power flush may be recommended. This involves connecting specialist equipment to the system to dislodge and flush out debris under pressure. A magnetic system filter fitted afterwards helps prevent future build-up.
- Have the NTC return sensor tested and replaced if faulty Gas Safe engineer
The engineer will use a multimeter to check the resistance of the return temperature sensor against expected values at a known water temperature. A sensor reading outside its normal range will need replacing. This is a straightforward parts-and-labour job for a qualified engineer.
- Request a PCB inspection if the fault persists Gas Safe engineer
If the pump, sensors, and system condition are all found to be acceptable but fault 106 keeps returning, the PCB may be misprocessing sensor data. The engineer will inspect the board for physical damage, check connections, and may carry out component-level testing. A faulty PCB will need to be replaced with a compatible unit.
- Call a Gas Safe registered engineer Gas Safe engineer
If the single reset and radiator bleeding have not resolved the fault, book a Gas Safe registered engineer to carry out a full diagnosis. Fault 106 indicates a condition that can lead to boiler damage if ignored, and the likely causes — pump failure, sludge blockage, or sensor faults — all require professional assessment and repair. You can find Gas Safe registered engineers at gassaferegister.co.uk.
Parts you may need
- Central heating circulation pump (compatible replacement) · from £85
- NTC return temperature sensor · from £25
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £70
- PCB (main control 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 £150–£400, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my Ariston boiler while fault code 106 is showing?
No — you should turn the boiler off and not continue running it with this fault active. Code 106 means the boiler has detected a potentially dangerous circulation problem and has locked out as a safety measure. Running it repeatedly by resetting it risks overheating the heat exchanger, which can cause expensive damage or, in a worst case, create a safety hazard. Get it diagnosed promptly.
Can I fix Ariston fault code 106 myself?
There are a couple of checks you can safely do yourself: bleed the radiators to remove trapped air, check and top up the system pressure to 1–1.5 bar, and try one reset. However, the most common causes — a failed circulation pump, sludge blockage, or a faulty sensor — all require a Gas Safe registered engineer to diagnose and repair. Do not attempt to open the boiler casing or work on internal components yourself.
How much does it cost to fix fault code 106 on an Ariston boiler?
For the most common repairs, expect to pay roughly £150–£400 including parts and labour. A circulation pump replacement typically falls in the £150–£300 range, while a sensor swap is usually £100–£200. A power flush to clear sludge can cost £300–£500 depending on system size. PCB replacement is less common but more expensive — often in the region of £400–£600 once parts and labour are included — so if that is identified as the cause, it is worth comparing the cost against a new boiler, particularly if yours is over 10 years old.
Why does the return temperature read higher than the flow temperature on my Ariston boiler?
Under normal operation, water leaves the boiler hot (the flow) and returns cooler after giving up heat to the radiators (the return). The return cannot physically be hotter than the flow unless something is wrong. The two most likely explanations are: water has almost stopped moving through the system (so localised heat builds up and confuses the sensors), or the return sensor itself is faulty and giving an incorrect high reading. Either way, fault code 106 is the boiler correctly identifying that something is not right and shutting down to protect itself.