Ariston 304 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 304 fault code mean?
Ariston fault code 304 is a safety lockout triggered when the boiler has been reset too many times in a short period. Specifically, Ariston's design allows up to five resets within any 15-minute window — if a sixth reset is attempted within that same window, the boiler locks out and displays code 304. This is the boiler's way of telling you that repeatedly pressing reset is not solving an underlying problem, and that continuing to do so would be unsafe. Note: on certain Ariston model ranges, code 304 has also been documented as a domestic hot water (DHW) temperature sensor fault — where the sensor responsible for measuring hot water temperature is reading outside its expected range. If your boiler has not been repeatedly reset and is showing 304, it is worth considering the DHW sensor as a possible cause. In the vast majority of cases, however, 304 indicates the too-many-resets lockout described above.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Recurring underlying fault triggering repeated resets Common
Code 304 is almost always a secondary fault. Something else — low pressure, ignition failure, a sensor error — has been causing the boiler to stop repeatedly, and the user (or an automated reset sequence) has hit reset too many times in quick succession. The 304 code is the result of that pattern, not the root cause itself. Watch the display carefully for any other fault codes appearing before or alongside 304.
- Low system water pressure Common
If the boiler's water pressure keeps dropping below the acceptable range (typically 1–1.5 bar), the boiler will fault and need resetting each time. This cycle of faulting and resetting is a very common route to triggering the 304 lockout. Check the pressure gauge on the boiler — if it reads below 1 bar, the system may need topping up via the filling loop.
- Gas supply interruption or ignition failure Common
A temporary interruption to the gas supply, or a fault with the ignition components, can cause the boiler to fail to light repeatedly. Each failed ignition attempt may require a reset, and if this happens frequently enough the 304 lockout follows. Ignition electrodes and flame sensors that are dirty or worn are a common culprit on the E-Combi One range in particular.
- DHW temperature sensor fault (model-specific) Sometimes
On some Ariston model ranges, code 304 has been documented as a domestic hot water temperature sensor fault, where the sensor is returning readings outside its expected parameters. If your boiler has not been repeatedly reset and 304 appears, this is worth investigating. A Gas Safe engineer can test the sensor's resistance with a multimeter to confirm whether it has failed.
- Air in the system Sometimes
Air trapped in the central heating system can cause erratic boiler behaviour, leading to repeated lockouts and resets. This is particularly likely after recent work on the system or after topping up the water pressure. Bleeding the radiators can resolve this.
How to fix it
- Stop pressing the reset button DIY safe
Further reset attempts will not clear a 304 lockout — the boiler has specifically locked out because too many resets have already been made. Pressing reset again will have no effect and may complicate diagnosis.
- Power-cycle the boiler at the mains DIY safe
Switch the boiler off at the mains isolator switch (or unplug it if accessible), wait at least two to three minutes, then switch it back on. This is the only way to clear the 304 lockout — a standard reset button press will not work once this threshold has been reached.
- Check your system water pressure DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, top up the pressure using the filling loop (the braided hose beneath the boiler). Stop adding water once the gauge reaches around 1.2–1.5 bar and do not overfill. If the pressure keeps dropping, there is a leak somewhere in the system that will need professional attention.
- Confirm your gas supply is working DIY safe
Check that another gas appliance in your home — a hob or gas fire, for example — is working normally. If no gas appliances are working, contact your gas supplier. If the boiler alone is affected, the gas supply to the boiler is likely not the issue.
- Bleed the radiators if you recently topped up the pressure DIY safe
If you have just added water to the system, air may have entered and is causing erratic boiler behaviour. Turn off the boiler and, using a radiator bleed key, open the bleed valve on each radiator slightly until water (not air) comes out, then close it again. Recheck the system pressure afterwards and top up if needed.
- Note any other fault codes on the display DIY safe
If the boiler fires up after the power-cycle but then faults again, take note of any code shown before the 304 reappears. That code is the real underlying fault. Having this information ready will help a Gas Safe engineer diagnose the root cause much more quickly.
- Call a Gas Safe registered engineer to diagnose the underlying fault Gas Safe engineer
If the boiler locks out again after the power-cycle, or if you cannot identify a simple cause such as low pressure, you need a Gas Safe registered engineer to investigate. They will be able to read the boiler's fault history, test the ignition components, gas valve, sensors, and PCB, and carry out any repairs safely and legally. Do not attempt to access internal components yourself.
Parts you may need
- Ignition electrode · from £35
- Flame sensing electrode · from £30
- DHW temperature sensor (NTC) · from £25
- PCB (printed circuit board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£400, depending on the underlying cause.
Frequently asked questions
Why won't the reset button clear my Ariston 304 fault?
Code 304 is specifically a lockout caused by too many reset attempts. Once this threshold is reached, pressing the reset button again has no effect — the boiler's software ignores further resets as a safety measure. The only way to clear the lockout is to switch the boiler off at the mains isolator, wait a couple of minutes, and switch it back on. However, if the underlying fault that was causing all those resets has not been fixed, the boiler is likely to fault again shortly afterwards.
What is actually wrong with my boiler if it shows code 304?
The 304 code itself just means the reset limit was reached — it does not tell you what the original problem was. The real fault could be anything from low system pressure and a gas supply issue, to a failing ignition electrode, a faulty sensor, or a problem with the PCB. Watch the display carefully: if another fault code appears before 304, that is your clue. A Gas Safe engineer can read the boiler's internal fault log to identify what has been happening.
Could code 304 mean a hot water sensor fault on my Ariston?
Yes, on certain Ariston model ranges code 304 has been documented as a domestic hot water (DHW) temperature sensor fault rather than a reset-lockout. If your boiler has not been repeatedly reset and is showing 304, a faulty or out-of-range DHW NTC sensor is worth considering. A Gas Safe engineer can check the sensor's resistance with a multimeter — a replacement sensor typically costs around £25–£40 for the part, with labour on top.
Is my Ariston boiler still under warranty — could this be a free repair?
Ariston offers manufacturer warranties ranging from 2 to 12 years depending on the model and registration. If your boiler is still within its warranty period, repairs to covered components — including faulty sensors and PCBs — may be carried out free of charge. Check your paperwork or the Ariston website to confirm your warranty status before paying for any repair. If the PCB has failed, replacement can cost upwards of £400–£500 including labour, so a valid warranty makes a significant difference.