Ariston boilers this guide covers
Our Ariston pages are written from Ariston's UK manuals: the Clas One and Clas System One user manual from Ariston Thermo UK, and the E-Combi One LPG installation and servicing instructions for Great Britain and Ireland. The Alteas One, Genus One and Alteas One Net use the same numbering for the codes we have checked, but your boiler's own manual is the final word.
How Ariston codes appear on the display
Ariston manuals print every code with a space after the first digit, such as 1 08 or 5 01, so 108 and 1 08 are the same code. The first digit tells you which part of the boiler raised it.
| First digit | Part of the boiler | Example from the manuals |
|---|---|---|
| 1 | Primary (central heating) circuit | 1 08 insufficient water, 1 09 pressure above 3 bar |
| 2 | Domestic hot water circuit | 2 05 hot water inlet probe open circuit |
| 3 | Internal electronics (main PCB) | 3 04 too many resets in 15 minutes |
| 4 | External electronics | 4 11 room sensor fault, zone 1 |
| 5 | Ignition and flame detection | 5 01 no flame detected |
| 6 | Air inlet and flue gas outlet | Fan and flue related codes |
| 7 | Multi-zone heating (optional zone modules) | 7 50 all heating zones locked |
A letter P in the middle marks a warning, so 5 P1 is the first warning in the ignition group. On the display the 5 looks like a capital S, which is why owners search for SP1 or SP2. SP1 and 5P1 are the same code.
Faults come in two types. A safety shut-off clears by itself once the cause has gone. A shutdown stays until you press RESET, and the display shows a symbol asking you to.
Ariston fault codes we cover
- Ariston error 108: not enough water in the heating circuit, usually fixed with the filling loop.
- Ariston SP1 (5 P1): the first ignition attempt failed and the boiler will try again.
- Ariston SP2 (5 P2): the second ignition attempt failed as well.
- Ariston error 501: no flame detected, a lockout that needs the RESET button.
- Ariston error 304: more than five resets in 15 minutes, so the boiler has stopped accepting them.
How to reset an Ariston boiler safely
Clas One and E-Combi One models have a RESET button on the control panel. First check that other gas appliances work, that a prepayment meter has credit and that the pressure is in range. Press RESET once and give the boiler time to light. Ariston allows five resets in 15 minutes, and a sixth brings up 3 04. We suggest stopping after two or three. If the code keeps returning, it needs an engineer.
Checking and topping up the pressure
Check the pressure with the heating cold and aim for 1 to 1.5 bar. To top up, the Clas One user manual describes connecting the filling loop's flexible hose under the boiler, opening its two taps slowly and closing them at the target pressure. The boiler then runs an air purge and shows PURGE for about seven minutes. Frequent top-ups point to a leak. Our low boiler pressure guide has more detail.
When to call a Gas Safe registered engineer
Call a Gas Safe registered engineer for any flame code that returns after two or three resets, any 3 xx electronics code apart from 3 04, and any fan or flue code. Never remove the casing. If you smell gas, turn off the supply at the meter if safe and call the National Gas Emergency Service on 0800 111 999. The Clas One user manual asks for a yearly service by a Gas Safe registered engineer and warns that skipping it invalidates the warranty.