Ariston 103 fault codewhat it means and how to fix it
The appliance is reporting insufficient circulation — water is not moving through it fast enough. Worth acting on before it becomes an overheat.
Take care
What 103 means
Ariston use a block of five codes, 103, 104, 105, 106 and 107, for insufficient circulation — one description covering all of them in their table, so the number itself does not narrow anything down. The milder relative is the warning 1P2, which Ariston call a failed flow check: the appliance is telling you it could not prove flow while carrying on running. The appliance works out flow indirectly by watching how quickly the temperature difference between flow and return develops when the burner fires. Water moving properly carries heat away and the two readings stay close together; water barely moving produces a rapid, wide split.
When that split exceeds what Ariston consider safe, the appliance reports this group of codes and reduces or stops firing. The valuable thing about seeing 103 rather than 101 is that you have caught the problem at the stage where it is still a circulation fault rather than an overheat. The same causes underlie both, but at this point the heat exchanger has not been repeatedly cooked. Acting now is materially cheaper than acting after two more weeks of resets.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump not achieving proper flow | Seized, worn, or set to too low a speed. On modern variable-speed pumps a failing electronic module can also hold the speed down. | Common |
| Air in the system | Frequently follows a refill done too quickly. Air pockets break flow and the appliance sees the temperature split immediately, which is why Ariston provide a deaeration cycle for use after filling. | Common |
| System heavily sludged | Magnetite narrows the waterways in the heat exchanger and the radiators. Common on systems over ten years old with no inhibitor and no filter. | Occasional |
| Bypass incorrectly set | An automatic bypass valve set wrong, or none fitted where the system needs one, so there is no flow path when the thermostatic valves close. | Occasional |
| Flow or return sensor misreading | The circulation is fine and the measurement is wrong. Worth eliminating, but not where an engineer will begin. | Rare |
What to try yourself
Check the pressure is 1 to 1.5 bar cold, which is Ariston's own range, and top up through the filling loop if it is below that.
Open all thermostatic radiator valves fully so the system has somewhere to circulate, and check any service valves below the appliance are open.
Bleed the radiators, working from the ground floor upwards and finishing with the highest one in the property. Top the pressure back up afterwards.
Run the heating for half an hour and note which radiators are cold and whereabouts on the panel. That pattern is the most useful thing you can hand an engineer.
If the code persists after bleeding and repressurising, book an engineer while it is still a circulation fault and not yet an overheat.
Get a Gas Safe registered engineer to look at your Ariston 103
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£120–£750
Time on site
45 minutes
Bleeding and repressurising costs nothing. A pump is £220 to £420 fitted. A chemical clean with a filter fitted is £400 to £750.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
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Related Ariston fault codes
- 101Get it checkedThe heating circuit has overheated and the appliance has shut down. The usual reason is that water has stopped moving, not that it is firing too hard.Related: overheat and circulation
- 501Get it checkedNo flame was detected during ignition and the appliance has locked out. Check the gas supply and the condensate pipe before booking anyone.Related: same brand
- 108Usually simple to fixThe heating circuit does not contain enough water to run safely. Refill through the filling loop to about 1.2 bar and the code clears.Related: same brand
- 604Get it checkedFan speed is outside the range the appliance expects, so it will not allow the burner to light. This one needs an engineer.Related: same brand
- 303Get it checkedThe main control board has reported an internal fault. It needs an engineer, and it is worth checking the electrical supply before assuming the board is dead.Related: same brand
- 109Get it checkedSystem pressure has gone above 3 bar. Usually this means somebody overfilled it, or the expansion vessel has failed.Related: same brand
Own an Ariston boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.