Ariston 303 fault codewhat it means and how to fix it
The 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.
Take care
Get a Gas Safe registered engineer to look at your Ariston 303
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 303 means
Codes beginning with 3 are what Ariston call the internal electronic part, and their entry against 303 is simply "main P.C.B.". The neighbouring numbers are worth knowing because they are not interchangeable: 301 is an EEPROM error, 302 a communication error, 304 too many resets, and 305, 306 and 307 are further main PCB entries.
A board fault is genuinely a board fault more often than not, but unstable mains supply produces board errors that look identical from outside, and a property with a marginal supply, a shared meter arrangement or a history of brownouts can throw one without anything being wrong with the board.
Ariston do not publish a voltage code for the boiler to tell you that, which is exactly why the supply is worth eliminating before a board is condemned. It is also worth checking whether the appliance has been reset more than five times in fifteen minutes, because that produces a 304 rather than a 303 and is not a fault at all. Replacing a control board is one of the more expensive repairs on a domestic boiler, so it is reasonable to expect an engineer to eliminate the supply first.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Control board component failure | Boards fail with age, heat cycling, and damp. The most likely explanation, particularly on an appliance over ten years old. | Common |
| Unstable or low mains supply | Voltage sag or a poor neutral produces board errors indistinguishable from a hardware failure, and there is no separate code that would tell you which you have. | Occasional |
| Moisture inside the case | A leak from the appliance itself, or condensation in an unheated garage installation, tracking across the board. | Occasional |
| Damaged wiring harness | A chafed or corroded connector making the board read impossible values from its own inputs. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£550
A power cycle costs nothing. A diagnostic visit is £80 to £140. A replacement main PCB is typically £300 to £520 fitted, which on an older appliance is worth weighing against replacement.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Ariston fault codes
- 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: ignition and gas supply
- 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
- 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: same brand
- 103Get it checkedThe appliance is reporting insufficient circulation — water is not moving through it fast enough. Worth acting on before it becomes an overheat.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
- 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.