What Ariston error 304 means
Error 304 is a safety limit on resets. Ariston's E-Combi One LPG design, installation and servicing instructions for Great Britain and Ireland, and the Clas One installation and servicing instructions for the 18 to 35 models, both list 3 04 as "Too many (> 5) resets in 15 minutes". Ariston's Spanish help pages describe it the same way, as too many restart attempts.
The E-Combi One LPG manual explains how it works. The boiler allows five resets in 15 minutes. On the sixth attempt it shuts down and can only operate again after the electricity supply has been disconnected. The Clas One and Clas System One user manual gives the same limit of five resets in 15 minutes and calls it a safety feature.
Why it is listed as an electronics code
Ariston's codes are grouped by their first digit, and the 3 group covers the boiler's internal electronics, alongside main PCB, EEPROM and communication errors. 304 sits there because it is the control board that counts the resets and enforces the limit. Unlike 3 01 to 3 03 and 3 05 to 3 07, it does not mean the board has failed. It means the board is doing its job.
How to clear error 304
Pressing RESET again will not help, because the boiler is refusing resets. The manual says the electricity supply has to be disconnected.
- Switch the boiler off at its external electrical switch. This is usually a fused spur on the wall near the boiler.
- Leave it off for a short while, then switch it back on.
- Let the boiler start up and watch the display.
- If the original fault code returns, do not start another round of resets. Work out what that code needs first.
If you cannot find a switch for the boiler, or it is wired in a way you do not recognise, do not take anything apart. A Gas Safe registered engineer can show you the isolation point during a visit.
Our general guide to resetting a boiler covers how resets work across brands and when to stop.
Find the code you were resetting
Nobody presses RESET six times without a reason. Something made the boiler lock out in the first place, and 304 has simply covered it up. Once power is back on, the original code will usually reappear if the cause is still there. These are the most likely culprits on the Ariston models we cover:
| Code you were resetting | What it means | What to do instead of resetting |
|---|---|---|
| 5 01 | No flame detected | Check gas, meter credit and LPG level, then call an engineer if it returns |
| 5 P1 or 5 P2 | Ignition attempt failed | These are warnings, so the boiler retries by itself without a reset |
| 1 08 | Insufficient water | Top up the pressure through the filling loop |
| 1 01 | Overheat | Call a Gas Safe registered engineer |
| 3 05 to 3 07 | Main PCB error | Call a Gas Safe registered engineer |
Low pressure is worth singling out. Error 108 clears once the water is topped up, and pressing RESET without topping up does nothing. If you were resetting a 108, a top-up is the fix.
A typical example
On a cold morning the boiler shows 501. Someone presses RESET, the boiler clicks, fails to light and shows 501 again. After the sixth press in quarter of an hour, 304 appears and RESET stops responding. The right sequence would have been to check the hob, the meter credit and the condensate pipe first, reset once or twice, and then book an engineer. Once 304 is cleared at the switch, that is still the order to follow.
How to avoid seeing 304 again
Ariston's limit of five resets is generous. We suggest stopping after two or three. If a code survives two or three resets with the obvious causes checked, more pressing will not fix it, and every failed ignition attempt is another chance for a condensate or flue problem to get worse. Keep a note of the original code, the time it appeared and anything you checked. The Clas One user manual says an isolated shutdown is not necessarily a problem, but frequent shutdowns should be reported to an authorised service centre.
When to call a Gas Safe registered engineer
Call a Gas Safe registered engineer if, after cutting and restoring the power, the boiler goes straight back into a lockout, or if 304 appears without anyone pressing RESET repeatedly. The second case is unusual and could point to a control or wiring problem inside the boiler. Never open the casing or try to wire around the switch.
Tell the engineer the original code, how many resets were tried and over what time, and whether the fault started after a power cut, cold weather or work on the gas supply.
Typical repair costs in the UK
304 itself costs nothing to clear. What you pay depends on the fault behind it.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Minor repair (small part, sensor, wiring) | Typically £100 to £180 |
| Ignition electrode | Typically £90 to £180 |
| PCB replacement, fitted | Typically £250 to £550 |