Ariston 504 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 504 fault code mean?
Fault code 504 on an Ariston boiler indicates a flame lift problem — the burner flame has become unstable or has gone out unexpectedly during normal operation, and the boiler's control system has detected this loss of stable combustion. Rather than continuing to run unsafely, the boiler locks out and displays 504 to alert you. On 24 kW models this code is closely linked to the preceding 5P3 error: the boiler records consecutive 5P3 ignition instability events, and once ten occur in a row the display escalates to 504. The root cause on those units is often a deteriorated burner gasket that reduces the electrical conductivity between the burner and its fixing screws — effectively fooling the control system into thinking the flame is lifting. Because flame instability directly affects combustion safety, this fault should be investigated promptly rather than reset repeatedly.
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
- Deteriorated Burner Gasket (24 kW Models) Common
On Ariston 24 kW boilers, the burner gasket degrades over time due to repeated heat cycling at low power and low fan speeds. As the gasket breaks down it insulates the burner from the fixing screws, reducing electrical conductivity and causing the control board to misread flame stability. This triggers repeated 5P3 events until the boiler escalates to fault 504. A small conductive plate fitted under one of the three burner fixing screws is the recognised fix for this specific failure mode.
- Faulty or Contaminated Ionisation Electrode (Flame Sensor) Common
The ionisation electrode passes a tiny electrical current through the flame to confirm its presence. If the tip is coated in carbon deposits, is cracked, or has drifted out of position, it sends a weak or intermittent signal — causing the boiler to believe the flame has lifted even when combustion is actually taking place. A clean or replacement electrode is often all that is needed.
- Intermittent or Low Gas Supply Common
If the gas pressure at the boiler drops — due to a partly closed isolation valve, high demand on the network, or a prepayment meter that has run out of credit — the burner cannot sustain a stable flame and the boiler trips on 504. This is one of the few causes a homeowner can quickly rule out themselves.
- Flue or Air Supply Obstruction Sometimes
A blocked or restricted flue prevents combustion gases escaping efficiently, disrupting the air-to-gas ratio inside the burner and causing the flame to become unstable. Birds nesting in the terminal, ice forming over the flue outlet in winter, or debris around the air intake can all trigger this fault.
- Faulty or Sticking Gas Valve Sometimes
The gas valve regulates fuel flow to the burner. If it sticks, drifts out of calibration, or fails to open and close at the correct rate, gas delivery becomes erratic and the flame lifts or extinguishes. Gas valve diagnosis and replacement must be carried out by a Gas Safe engineer using specialist equipment.
- Blocked or Corroded Burner Sometimes
Over many years of operation, debris, corrosion products, or scale can partially block the small ports in the burner through which gas flows. Restricted ports produce an uneven, weakened flame that is prone to lifting, particularly at low modulation.
- PCB or Electronic Fault Rare
The printed circuit board interprets signals from the ionisation electrode and controls the gas valve. Known failure modes include cold solder joints, failing electrolytic or ceramic capacitors, and overheating resistors near the gas valve driver transistors. A faulty PCB may misinterpret a perfectly stable flame as having lifted, or may itself be causing erratic gas valve behaviour.
How to fix it
- Check that your gas supply is working DIY safe
Turn on a gas hob burner or check another gas appliance. If you have a prepayment meter, confirm it is in credit and the supply has not been interrupted. If no gas appliances work, contact your gas supplier rather than the boiler engineer.
- Reset the boiler once or twice DIY safe
Locate the reset button on your Ariston boiler (usually marked with a flame symbol or labelled 'Reset') and hold it for up to 10 seconds. If the boiler fires up and runs normally, monitor it for the next few hours. Limit resets to two or three attempts — repeatedly resetting a boiler with an underlying combustion fault can stress components and may invalidate your warranty.
- Check for obvious flue obstructions DIY safe
Go outside and look at the flue terminal on the exterior wall or roof. Confirm it is not blocked by ice, debris, a bird nest, or anything that has fallen against it. Do not attempt to dismantle the flue — just a visual check from a safe distance. If you spot a blockage you can safely remove (such as leaf debris around an external grille), do so, then reset the boiler.
- Do not attempt to inspect or clean the burner, gas valve, or ionisation electrode yourself Gas Safe engineer
All work on the burner assembly, flame sensor, gas valve, and combustion components must be carried out by a Gas Safe registered engineer. This applies to cleaning the ionisation electrode, adjusting burner fixing screws, fitting the conductive plate to resolve the 5P3/504 gasket fault on 24 kW models, and any gas valve testing or replacement.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
Provide the engineer with the exact code shown (504, or 5P3 escalating to 504) and note how many times it has recurred. A qualified engineer will inspect the ionisation electrode, burner gasket condition, gas pressure and valve operation, flue integrity, and PCB health. On 24 kW Ariston models they should be aware of the specific gasket-conductivity fault and the conductive plate fix.
Parts you may need
- Ionisation electrode (flame sensor) · from £25
- Burner gasket · from £18
- Burner assembly · from £85
- Gas valve · from £130
- PCB (printed circuit board) · from £220
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 £90–£350, depending on the underlying cause.
Frequently asked questions
What is the difference between Ariston fault codes 5P3 and 504?
Code 5P3 is an ignition instability warning that the boiler logs each time the flame behaves erratically. On 24 kW Ariston models in particular, this is often caused by a degraded burner gasket reducing conductivity between the burner and its fixing screws. Once the boiler records ten consecutive 5P3 faults it escalates to a full lockout and shows 504. If your boiler keeps returning to 5P3 before eventually settling on 504, the burner gasket or ionisation electrode is the most likely culprit and an engineer should inspect both.
Can I fix Ariston fault code 504 myself?
The two checks a homeowner can safely perform are confirming the gas supply is on and resetting the boiler once or twice. Everything else — cleaning or replacing the ionisation electrode, inspecting the burner and gasket, testing the gas valve, or working on the PCB — must be done by a Gas Safe registered engineer. Gas component work is a legal requirement under UK gas safety regulations, not just a recommendation.
How much does it typically cost to repair an Ariston 504 fault in the UK?
Most people pay between £90 and £350 all-in, depending on what needs replacing. A straightforward ionisation electrode clean or replacement tends to sit at the lower end (around £90–£150 including labour). A gas valve swap usually comes to £200–£320. If the PCB is at fault, costs can reach £350–£500 or more depending on the model — in that situation, weigh the repair cost against the age of the boiler. If yours is over ten years old and needs an expensive PCB, a new boiler may be the more sensible long-term investment.
Why does my Ariston 24 kW boiler keep getting fault code 504?
Repeated 504 faults on Ariston 24 kW models are frequently traced to the burner gasket issue described above. These boilers run at low modulation for long periods, which subjects the burner gasket to repeated heat stress. Over time the gasket deteriorates and electrically isolates the burner body from its fixing screws, causing the control board to misread flame conductivity. The recognised fix is fitting a small conductive plate under one of the three burner fixing screws — a quick job for a Gas Safe engineer who is familiar with this fault.