Ariston 107 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 107 fault code mean?
Fault code 107 on Ariston boilers signals an 'Insufficient Circulation' condition. The boiler's onboard monitoring has detected that the return pipe temperature is running more than 30°C higher than the flow pipe temperature — a physically abnormal situation, since the water returning to the boiler should always be cooler than the water leaving it. When this threshold is crossed, the boiler locks out as a safety precaution to prevent overheating and internal damage. The root cause is either a genuine circulation problem (water is not moving freely around the system) or a faulty temperature sensor giving a misleading reading. This fault code applies across the full range of Ariston boiler models.
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 failing circulation pump Common
The pump is responsible for pushing heated water around your radiators and back to the boiler. If it seizes, runs too slowly, or fails entirely, water movement stalls and heat builds up unevenly — exactly the imbalance that triggers code 107. This is the most common culprit engineers find on this fault.
- Sludge or debris blocking the system Common
Over time, rust particles and scale accumulate inside pipework and radiators, forming a thick sludge that restricts water flow. Partial blockages reduce circulation enough to create significant temperature differences between the flow and return, causing the boiler to flag insufficient circulation.
- Air trapped in the circulation pump or pipework Sometimes
A pocket of air lodged in the pump head or pipework prevents proper water flow. The pump may be running but moving very little water, leading to localised overheating and an abnormal temperature differential between flow and return pipes.
- Faulty NTC temperature sensor on the return pipe Sometimes
If the thermistor measuring return pipe temperature has drifted out of calibration or developed an intermittent fault, it can report an artificially high reading that exceeds the flow temperature by 30°C — triggering code 107 even when circulation is perfectly normal.
- PCB fault or wiring issue Rare
In rarer cases, a damaged printed circuit board or a loose/corroded connection between the PCB and the temperature sensors can produce erroneous readings that mimic a circulation fault. The boiler reacts to data that does not reflect actual system conditions.
How to fix it
- Turn off the boiler and let it cool briefly DIY safe
If fault code 107 has appeared, switch the boiler off at the programmer or room thermostat. Allow five to ten minutes for the system to settle before doing anything else. Running the boiler repeatedly while this fault is active risks heat exchanger damage.
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it has dropped below 0.8 bar, low water volume can contribute to poor circulation. Top up via the filling loop following your boiler's handbook, then re-check. Do not overfill beyond 2.0 bar.
- Bleed your radiators to release trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator key to open each bleed valve slightly until water — not air — trickles out, then close it. Work back towards the boiler. After bleeding, re-check the system pressure and top up again if it has dropped below 1.0 bar.
- Attempt a single boiler reset DIY safe
With the system bled and pressure correct, press and hold the boiler's reset button for approximately five seconds. If the fault clears and the boiler fires up normally, monitor it over the next hour or two. If code 107 returns, do not keep resetting — repeated resets on a genuine circulation fault can cause further damage and will not resolve the underlying problem.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the fault reappears after one reset — or if you are unsure about any of the steps above — contact a Gas Safe registered engineer. They will carry out a full assessment: testing the circulation pump's speed and output, inspecting the flow and return NTC sensors with a multimeter, checking for sludge contamination, and examining PCB connections. Depending on what they find, work may include pump replacement, a power flush or chemical flush of the system, sensor replacement, or PCB investigation. Only a qualified engineer should carry out any of this work.
Parts you may need
- Circulation pump (compatible replacement) · from £120
- NTC return pipe temperature sensor · from £35
- Magnetic system filter · from £75
- Central heating inhibitor (1 litre) · from £15
- Ariston PCB (model-specific) · from £220
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 keep using my Ariston boiler when fault code 107 is showing?
No — you should turn the boiler off until the fault is resolved. Code 107 indicates the boiler has detected an unsafe temperature imbalance between the flow and return pipes. Continuing to operate it risks overheating the heat exchanger, which can cause costly internal damage. One careful reset attempt is reasonable, but if the code comes straight back, leave the boiler off and call an engineer.
How much does it cost to fix Ariston fault code 107 in the UK?
The most common repair — circulation pump replacement — typically costs between £150 and £300 all-in, covering the pump and an engineer's labour. If the system is contaminated with sludge, a chemical flush adds around £100–£200, while a full power flush for a heavily affected system can cost £350–£850 depending on the number of radiators. A faulty return temperature sensor is usually the cheapest fix at roughly £100–£200 including labour. In the rare event the PCB needs replacing, costs can reach £400–£550, but this is not the typical outcome for fault 107.
Why is my Ariston boiler showing code 107 even though the heating seemed to be working fine?
Code 107 can be triggered by a faulty NTC temperature sensor on the return pipe rather than a genuine circulation failure. If the sensor has drifted out of range, it can report a falsely high return temperature that exceeds the flow reading by the 30°C threshold — locking the boiler out even when water is circulating normally. An engineer can test the sensor's resistance with a multimeter and replace it if it is out of specification, which is a relatively straightforward and affordable repair.
Will a power flush fix Ariston fault code 107 caused by sludge?
A power flush is very effective when sludge is the root cause, as it uses specialist equipment to push water and cleaning chemicals through the system at high velocity, dislodging and flushing out accumulated debris. For milder contamination, a chemical flush (where descaler and inhibitor are added to the system without specialist machinery) can achieve good results at a lower cost. After either treatment, fitting a magnetic filter helps prevent sludge from building up again and is strongly recommended.