- Boilers
- Gas boiler
304
Ariston 304 on a gas boiler: too many resets in 15 minutes
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Ariston 304 means RESET has been pressed more than 5 times in 15 minutes and the boiler has locked out. Disconnect it from the power to unlock it, write down which code it showed before and, if that returns, call the technical service.
Safety first
Stop and check this first
- If you smell gas: Ariston says to close the gas tap, open the windows and call the technician. Do not touch switches, plugs or anything that sparks, leave the home and call your gas distributor's emergency number from the street.
- Do not chain resets: the limit of 5 in 15 minutes is there for safety. If the lockout repeats often, Ariston asks for an authorised technical service visit.
Safe checks you can perform
- Code shown before 304
- Gas tap and water pressure
- One unlock by disconnecting the boiler
- Error history and diagnosis by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Error history reading
- Do not attempt this professional task: Gas or flame failure safety check
- Do not attempt this professional task: Combustion analysis after the repair
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Error history reading
What they doThe technician opens the error history menu (last 10, Err 0 to Err 9) and notes the code that caused the lockouts.
They must show youthe error list with the code that repeated.
In writingwhich code caused the lockouts and what was done to fix it.
Gas or flame failure safety check
What they doIf the underlying lockout was a flame fault, the technician reads the ionisation current in the boiler menu and checks the gas or flame failure safety, as Ariston asks during maintenance.
They must show youthe ionisation current measured against the manufacturer's specification.
In writingthe value, the part checked or replaced and why.
Combustion analysis after the repair
What they doThe technician carries out the combustion analysis Ariston recommends doing periodically and checks the flue system is sealed.
They must show youthe analyser printout.
In writingthe values measured against the manufacturer's specification.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Code shown before 304
- Who:
- you.
- How:
- remember, or check your photos or notes, which code was flashing before you pressed RESET several times (for example 501 or 108).
- Result that matters:
- knowing which lockout was repeating.
- If you know it:
- tell the technician; that is the fault to diagnose, not 304.
- If you do not know it:
- the technician can see it in the boiler's history of the last 10 errors.
- Evidence to keep:
- a photo of the display with 304 and, if you have it, of the earlier code.
Evidence: Ariston CLAS One y CLAS One System, instrucciones técnicas para la instalación y el mantenimiento
Gas tap and water pressure
- Who:
- you.
- How:
- check that the boiler's gas tap is open and read the pressure gauge. Ariston says to top up with the filling tap under the boiler and close it once it reaches 1 to 1.5 bar.
- Result that matters:
- whether gas is reaching the boiler and whether the pressure is in that range.
- If the tap was closed or the pressure low:
- correct it before unlocking; it may explain the repeating lockout.
- If both are fine:
- the underlying lockout has another cause; go on to the unlock.
- Evidence to keep:
- a photo of the gauge and a note of the gas tap position.
Evidence: Calderas Ariston: significados de los códigos de error · Ariston CLAS One y CLAS One System, instrucciones técnicas para la instalación y el mantenimiento
One unlock by disconnecting the boiler
- Who:
- you.
- How:
- Ariston says that after the sixth attempt in 15 minutes the boiler can only be unlocked by disconnecting it. Turn the external electrical switch to OFF, switch it back on and let the boiler try to start. Do not press RESET repeatedly.
- Result that matters:
- whether it starts and runs without locking out again.
- If it runs:
- note the date and time; an isolated lockout is not a problem, according to Ariston.
- If the same lockout returns:
- stop; switch off, external switch to OFF, close the gas tap and call the technical service.
- Evidence to keep:
- a note of the time of each unlock and the code that comes back.
Evidence: Ariston CLAS One y CLAS One System, instrucciones técnicas para la instalación y el mantenimiento · Ariston CLAS One System, manual de usuario
Error history and diagnosis by the technician
- Who:
- Ariston official technical service technician.
- How:
- reads the boiler's history of the last 10 errors and diagnoses the code that kept repeating.
- Result that matters:
- which lockout caused the resets and why it repeats.
- If it is a flame fault:
- checks ignition, flame detection and gas.
- If it is another group (water, flue, board):
- checks that system.
- Evidence to keep:
- a job sheet with the code found in the history and what was repaired.
Ariston asks for the power to be cut at the external two-pole switch and the gas closed before the boiler is opened. The inside, burner, ignition and gas are technician-only work.
Evidence: Ariston CLAS One y CLAS One System, instrucciones técnicas para la instalación y el mantenimiento
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Ariston
- Product type
- Gas boiler
Known code variants
- 3 04
- Ariston 3 04
- Chaffoteaux 3 04
- Clas One 304
- Error 304 Ariston
- Error 304 Chaffoteaux
- Genus One 304
- Pigma 304
- Pigma Advance 304
What it means
In the CLAS One table, 3 04 belongs to group 3, internal electronics, and means too many reset attempts (more than 5) in 15 minutes. Ariston explains that non-volatile lockouts are cleared with the RESET button and that, for safety, the boiler allows at most 5 reactivations in 15 minutes; on the sixth attempt it locks and can only be unlocked by disconnecting it. 304 is not the fault itself: it says another lockout kept repeating, and that code is the one to diagnose.
Warnings and stop conditions
- Criticaleditorial warning
If you smell gas: Ariston says to close the gas tap, open the windows and call the technician. Do not touch switches, plugs or anything that sparks, leave the home and call your gas distributor's emergency number from the street.
Evidence: Ariston CLAS One y CLAS One System, instrucciones técnicas para la instalación y el mantenimiento · Ariston CLAS One System, manual de usuario
- Highsource verified
Do not chain resets: the limit of 5 in 15 minutes is there for safety. If the lockout repeats often, Ariston asks for an authorised technical service visit.
Evidence: Ariston CLAS One y CLAS One System, instrucciones técnicas para la instalación y el mantenimiento · Ariston CLAS One System, manual de usuario
Symptoms and causes
Probable causes · in order
- An underlying lockout that keeps repeatingCommon
The usual case: a non-volatile error, for example 501 no flame, returns after every RESET. 304 hides that code, which is the real fault to find.
Evidence: Calderas Ariston: significados de los códigos de error · Ariston CLAS One y CLAS One System, instrucciones técnicas para la instalación y el mantenimiento
- Several resets in a row in one episodePossible
Fits if, after disconnecting the boiler, it starts and does not lock out again. Ariston says a sporadic or isolated lockout is not a problem.
Evidence: Ariston CLAS One y CLAS One System, instrucciones técnicas para la instalación y el mantenimiento · Ariston CLAS One System, manual de usuario
Virtual technician
Does 304 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Calderas Ariston: significados de los códigos de error
Ariston · 2025 · official_doc
- 3
Ariston CLAS One System, manual de usuario
Ariston · 2017 · manual
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