Ariston Fault Code 501: No Flame Detected
Fault code 501 on an Ariston boiler means the boiler attempted to light the burner but received no confirmation that a flame was established. As a safety measure, the boiler immediately locks out and shuts down, leaving you without heating or hot water until the underlying problem is resolved. On some older Ariston models you may see this displayed as SP1 or 5P1 rather than 501 — the cause and remedy are the same.
General guidance only — not a substitute for professional advice. 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.
What does the Ariston 501 fault code mean?
Fault code 501 on an Ariston boiler means the boiler attempted to light the burner but received no confirmation that a flame was established. As a safety measure, the boiler immediately locks out and shuts down, leaving you without heating or hot water until the underlying problem is resolved. On some older Ariston models you may see this displayed as SP1 or 5P1 rather than 501 — the cause and remedy are the same.
Common causes
- Interrupted or absent gas supply Common
If the gas meter valve has been accidentally closed, your credit has run out on a prepayment meter, or there is a wider supply interruption in your area, the boiler cannot ignite. This is the first thing worth ruling out because it requires no specialist knowledge to check.
- Worn or dirty ignition electrode Common
The ignition electrode produces the spark that lights the burner. Over time the tip can erode, become coated in carbon deposits, or crack, causing weak or absent sparks. This is one of the most frequent reasons a 501 fault appears after a boiler has been running for several years.
- Faulty or contaminated flame-sensing probe Common
Once the burner lights, a separate ionisation probe detects the flame and tells the PCB all is well. If this probe is dirty, misaligned, or has failed, the board never receives the 'flame confirmed' signal and triggers a 501 lockout even if ignition briefly occurred.
- Frozen or blocked condensate pipe Sometimes
In cold weather the plastic condensate pipe that runs outside can freeze solid. When the boiler cannot expel condensate it often fails to complete the ignition sequence, producing a 501 error. This is particularly common from late autumn through to early spring.
- Low gas pressure at the appliance Sometimes
Even when the supply is on, low gas pressure — caused by a partially closed internal service valve, a failing gas valve, or network pressure issues — can mean not enough gas reaches the burner for a reliable flame to form.
- Exhaust fan or flue obstruction Sometimes
The fan must reach the correct speed before the ignition sequence starts. A sluggish or failed fan, or a flue blocked by debris or a bird nest, can prevent the pre-purge cycle completing, causing a lockout before the spark is even attempted.
- Faulty gas valve Rare
The gas valve controls the flow of gas to the burner. If it sticks closed or opens only partially due to electrical or mechanical failure, insufficient fuel reaches the burner and no flame is detected.
- PCB (printed circuit board) failure Rare
The PCB orchestrates the entire ignition sequence. If it cannot send the correct signals to the electrode or read back the flame-sensor output, a 501 code results. PCB faults are less common but worth considering if all other components test out correctly.
How to fix it
- Check your gas supply is working DIY safe
Try another gas appliance in your home — a hob burner is ideal. If that also fails to light, the problem is with the gas supply rather than the boiler itself. Check that the isolation valve on the gas meter (the handle should be in line with the pipe, not across it) is fully open. If you suspect a supply outage, contact your gas supplier. Do not attempt to interfere with the meter or any pipework.
- Inspect the condensate pipe for freezing DIY safe
In cold weather, locate the white plastic pipe (usually 21.5 mm or 32 mm diameter) that exits your boiler and runs to an outside drain or soakaway. If it feels solid or you can hear gurgling that stops abruptly, it is likely frozen. Pour warm — not boiling — water along its length, or wrap it with a warm, damp cloth until it clears. Once thawed, reset the boiler as described in the next step.
- Check and correct the boiler pressure DIY safe
Look at the pressure gauge on the boiler fascia. It should read between 1.0 and 1.5 bar at rest. If it is below 1.0 bar, use the filling loop (a braided or rigid connection between the cold mains and the heating circuit, often under the boiler) to slowly top the system up to around 1.2 bar, then close the filling loop valves fully. If you are unsure how to locate the filling loop, consult your boiler manual or call an engineer.
- Reset the boiler DIY safe
Press and hold the reset button — typically marked with a flame or a circular arrow symbol — for around three seconds until the display changes. Allow the boiler two to three minutes to complete its ignition sequence. If it fires successfully, monitor it over the next hour. Limit yourself to two or three reset attempts in total; repeated resets without fixing the root cause will not help and may place extra strain on components.
- Arrange a Gas Safe engineer to inspect the ignition electrode and flame sensor Gas Safe engineer
If the fault persists after the checks above, the most likely culprits are the ignition electrode and the ionisation probe. A Gas Safe registered engineer will remove the burner assembly, inspect the electrode tip for wear or cracking, check the spark gap is within specification, clean or replace the flame-sensing probe, and verify all wiring connections are secure. Neither the electrode nor the probe should be touched by a homeowner — they sit within the gas-carrying section of the boiler.
- Have the engineer assess gas pressure and the gas valve Gas Safe engineer
The engineer will use a manometer to measure gas pressure at the boiler inlet and at the valve outlet. If inlet pressure is correct but the valve is not opening properly, the valve will need to be replaced. Gas valve work must only be carried out by a Gas Safe registered engineer — it is both dangerous and illegal for anyone else to do so.
- Request a flue and fan check Gas Safe engineer
As part of their investigation the engineer should verify that the flue is clear of obstructions and that the fan runs up to speed within the correct time window. A restricted flue or an underperforming fan will prevent the boiler completing its pre-purge and ignition cycles, and in the worst case could create a carbon monoxide risk.
- If all components check out, arrange a PCB assessment Gas Safe engineer
If the electrode, flame sensor, gas valve, fan, and gas supply are all confirmed as sound, the PCB may be sending incorrect signals or failing to register the flame confirmation. PCB diagnosis and replacement must be carried out by a Gas Safe registered engineer using manufacturer-approved parts and should be the last avenue explored, given the cost involved.
Parts you may need
- Ignition electrode · from £35
- Ionisation / flame-sensing probe · from £30
- Gas valve · from £120
- Flue fan / exhaust fan · from £90
- PCB (printed circuit board) · from £200
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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my Ariston boiler to clear the 501 code?
You can try two or three resets to see if the fault clears — for example after thawing a frozen condensate pipe or topping up low pressure. Beyond that, repeated resets are unlikely to help and may place unnecessary strain on the ignition components. If the 501 code returns after a couple of attempts, stop resetting and call a Gas Safe registered engineer to diagnose the root cause.
Is the Ariston 501 fault dangerous?
The lockout itself is a safety feature, so your boiler shutting down is the system working as intended. However, the underlying cause could involve a gas supply problem or a blocked flue, both of which carry risks if left uninvestigated. Do not attempt to bypass the lockout or tamper with any gas components. If you smell gas at any point, do not reset the boiler — leave the property, open windows if safe to do so, and call the National Gas Emergency Service on 0800 111 999 immediately.
How much does it cost to fix an Ariston 501 fault in the UK?
Most homeowners pay somewhere between £150 and £350 to resolve a 501 fault, depending on which component is at fault. A straightforward ignition electrode or flame-sensor replacement typically sits towards the lower end of that range, while a gas valve replacement can push towards the top. PCB replacements are the exception — these can cost £300–£500 or more and are not reflected in the typical repair range above. Getting a full diagnostic from the engineer before authorising work is always advisable.
How can I prevent the 501 fault from coming back?
The single most effective step is booking an annual boiler service with a Gas Safe registered engineer. During a service the engineer will clean the ignition electrode and flame sensor, check gas pressure, inspect the flue, and catch minor wear before it causes a breakdown. In cold weather, lagging any exposed sections of your condensate pipe with foam pipe insulation helps prevent winter freezing, which is one of the more common triggers for a 501 lockout between November and March.