Which faults are common on Ariston boilers?
Ariston is an Italian manufacturer whose Clas combis and system boilers are fitted in many UK homes. Its error codes follow a logical pattern: three digits, with the first telling you where the problem is. That makes Ariston codes easier to read than most once you know the system.
Water and pressure (1xx codes). 108 means there is not enough water in the system and it needs filling. Top up through the filling loop as your manual shows. If it comes back within days, there is a leak. 109 is the opposite, pressure above 3 bar, which can happen if a filling loop is left open. 101 is an overheat, and 103 to 107 mean water is not circulating well enough, usually because of air, sludge, closed valves or a pump problem. See boiler losing pressure and cold radiators.
Ignition and flame (5xx codes). 501 means no flame was detected. Check the gas supply first, including prepayment credit, and the condensate pipe in freezing weather. A 5 P1 or 5 P2 warning means a first or second ignition attempt failed; if the boiler then lights, it has recovered by itself. Our guide to a boiler that won’t fire up goes through the checks, and the frozen condensate pipe guide covers thawing.
Electronics (3xx codes). Most 3xx codes point to the control board. 304 is the one owners trigger themselves, by pressing reset more than five times in 15 minutes.
Why you should not keep pressing reset
Ariston builds a hard limit into its boilers: five resets in 15 minutes. After that the boiler shuts down and only runs again once the power has been disconnected. The limit exists because a fault that keeps returning has a real cause, and relighting a boiler again and again does not remove it. Reset once after the basic checks; if the code returns, stop.
Servicing an Ariston boiler
A yearly service by a Gas Safe registered engineer checks combustion, the flue, the electrodes and the condensate trap, and it usually keeps the manufacturer’s guarantee valid. See boiler servicing.
When should you get an engineer?
If a code returns after one reset, if you see 304, or if the boiler is losing pressure often, send an enquiry with the model name and the exact code. A local Gas Safe registered engineer will diagnose the fault and give a price before any work starts. If your Ariston is still under guarantee, contact Ariston first.