Ariston 5P3 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 5P3 fault code mean?
The 5P3 fault code on Ariston boilers indicates that the burner flame went out while the boiler was already running — as opposed to a failure to ignite in the first place. The boiler managed to light successfully, but something caused the flame to extinguish mid-operation, triggering a safety lockout. This code sits within the 5P1–5P3 family of flame-related faults on Ariston appliances: 5P1 typically relates to a failure to ignite on first attempt, 5P2 to repeated ignition failure, and 5P3 specifically to flame loss once the burner is already alight. The control board monitors the flame signal continuously via an ionisation probe; if that signal drops away unexpectedly, the board locks the boiler out and displays 5P3.
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
- Fluctuating or low gas supply pressure Common
If the gas pressure at the boiler drops during a heating or hot water demand, the flame can simply starve and extinguish. This may affect other gas appliances in the property simultaneously, or it may only be noticeable at the boiler. A Gas Safe engineer will measure the working gas pressure with a manometer to confirm this.
- Faulty ionisation (flame detection) probe Common
The ionisation probe generates a tiny electrical signal to confirm the burner is alight. If the probe is coated in carbon deposits, cracked, or positioned incorrectly, it may lose the flame signal even when the flame is still present, causing a spurious lockout. Cleaning or replacing the probe is usually the cheapest first fix an engineer will try.
- Air pressure switch or fan fault Common
The fan creates the draught that supports combustion, and the air pressure switch (APS) confirms that the fan is running correctly before and during burner operation. If the fan struggles, slows, or if the APS or its connecting rubber tubing develops a fault mid-cycle, the control board can interpret this as an unsafe condition and shut the burner down. On the E-Combi One 24 in particular, a rumbling noise accompanying the 5P3 code often points toward the fan assembly.
- Gas valve malfunction Sometimes
The gas valve controls the flow of gas to the burner. A valve that is sticking, has a failing solenoid coil, or is not receiving the correct signal from the PCB can restrict or cut gas flow during operation, causing flame loss. Gas valve work must only be carried out by a Gas Safe registered engineer.
- PCB fault — gas relay or transistor failure Sometimes
The PCB (printed circuit board) orchestrates the entire ignition and run sequence. On older Ariston Genus One models in particular, cold solder joints, failing capacitors, and overheating transistors that control the gas valve relay have been known to cause intermittent flame-loss faults. A PCB fault is worth considering when simpler components have been ruled out, especially on boilers more than five to eight years old.
- Ignition electrode or lead degradation Rare
Although 5P3 specifically involves flame loss rather than failure to ignite, a degraded spark electrode or a cracked ignition lead can produce a weak or unstable flame immediately after ignition that the ionisation probe fails to recognise as stable, resulting in a rapid shutdown logged as 5P3.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Ariston boiler's fascia — it is usually marked with a flame symbol or labelled 'Reset'. Press and hold it for approximately five to ten seconds until the boiler attempts to restart. Allow one reset attempt. If the 5P3 code returns, do not keep resetting repeatedly, as this will not resolve the underlying problem and may mask useful diagnostic information for an engineer.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home — your hob or gas fire, for example — are working normally. If they are not, the issue may be with your gas supply rather than the boiler itself. Contact your gas supplier or call the National Gas Emergency Service on 0800 111 999 if you suspect a supply interruption or smell gas.
- Check the boiler's system pressure DIY safe
Look at the pressure gauge on the boiler fascia. For most Ariston combi boilers the normal cold-system pressure is between 1.0 and 1.5 bar. If the needle is below 0.5 bar, low pressure could be contributing to erratic operation. You can top up pressure using the filling loop (usually a braided hose or lever under the boiler) — consult your boiler's user guide for the exact procedure, and stop as soon as the gauge reaches around 1.2–1.5 bar.
- Inspect the flue terminal and ventilation externally DIY safe
Go outside and check that the flue outlet (the white or grey plastic terminal on an external wall or roof) is not obstructed by debris, a bird nest, or ice. A blocked flue can disrupt combustion airflow and cause mid-operation shutdowns. Do not attempt to dismantle the flue — simply check for obvious external blockages.
- Have a Gas Safe engineer inspect the ionisation probe and ignition components Gas Safe engineer
An engineer should start with the simplest and most affordable components. The ionisation (flame detection) probe is the most common culprit for mid-run flame-loss faults and is inexpensive to clean or replace. The ignition electrode and lead should also be inspected for wear or cracking at the same visit.
- Have the engineer test the fan and air pressure switch Gas Safe engineer
The engineer should measure the fan's speed and current draw, check the air pressure switch for correct operation, and inspect the rubber tubing connecting the APS for splits or blockages. On the E-Combi One 24, a noisy fan alongside 5P3 is a strong indicator that the fan or APS is the root cause.
- Have the engineer check gas valve operation and working gas pressure Gas Safe engineer
Using a manometer, the engineer should confirm that the working gas pressure at the boiler is within the manufacturer's specification. They should also test the gas valve's solenoid coils and verify that the valve is opening and modulating correctly during a call for heat.
- Have the engineer assess the PCB if other components are healthy Gas Safe engineer
If the probe, electrodes, fan, APS, and gas valve are all functioning correctly, the PCB becomes the likely culprit — particularly on Genus One models over five years old where transistor and capacitor degradation is a known issue. Some specialist companies can repair rather than replace Ariston PCBs, which may reduce cost. On a boiler more than eight to ten years old, discuss with your engineer whether PCB replacement is economical compared with a new boiler.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All diagnostic and repair work beyond the homeowner checks above must be carried out by a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk. It is illegal under UK law for an unregistered person to work on a gas appliance, and doing so would also invalidate any remaining warranty and your home insurance.
Parts you may need
- Ionisation / flame detection probe · from £25
- Ignition electrode · from £20
- Ignition lead / HT lead · from £18
- Air pressure switch (APS) · from £45
- Boiler fan / flue fan assembly · from £95
- Gas valve · from £110
- 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 £150–£400, depending on the underlying cause.
Frequently asked questions
What is the difference between Ariston fault codes 5P1, 5P2, and 5P3?
All three codes relate to problems with the burner flame, but they occur at different stages. 5P1 typically means the boiler failed to ignite on its first attempt. 5P2 indicates repeated ignition failure across multiple tries. 5P3 is distinct because the boiler did ignite successfully — the flame then went out while the boiler was running. This timing is a useful clue for your engineer, as it narrows the likely causes toward components that affect combustion stability rather than initial spark generation.
Can I fix Ariston fault code 5P3 myself?
The only homeowner-safe actions are resetting the boiler once, checking that other gas appliances are working normally, topping up system pressure if it is low, and checking for an obvious external flue obstruction. All other diagnostic and repair work — including anything involving the gas valve, ionisation probe, fan, air pressure switch, or PCB — must be carried out by a Gas Safe registered engineer. Working on gas appliances without Gas Safe registration is illegal in the UK and is dangerous.
How much does it cost to fix a 5P3 fault on an Ariston boiler?
For the most common repairs — cleaning or replacing the ionisation probe, ignition electrode, or air pressure switch — most homeowners pay in the region of £150 to £250 once call-out, labour, and parts are included. A fan replacement typically costs £250 to £400. If the gas valve needs replacing the cost is usually around £180 to £250 all-in. PCB replacement is significantly more expensive at roughly £410 to £650 including parts and labour; on a boiler over eight to ten years old it is worth getting a new boiler quote alongside a PCB quote to make an informed decision.
My Ariston boiler shows 5P3 intermittently — it works fine for a while then locks out again. What does that suggest?
Intermittent 5P3 faults that come and go are a classic sign of either a marginal air pressure switch that cuts out under certain conditions (for example when the fan gets hot), a ionisation probe that is partially degraded and loses signal inconsistently, or early-stage PCB component failure — particularly a transistor or capacitor that behaves differently when warm. An engineer who can run the boiler through a heating cycle and monitor live data will be able to catch the fault as it happens, which makes diagnosis much easier. Mention the intermittent nature of the fault when you book the appointment.
Is it worth repairing a 5P3 fault or should I replace the boiler?
If the fix is a straightforward component such as the ionisation probe, ignition lead, or air pressure switch, repair is almost always the right choice regardless of boiler age. If the diagnosis points to a gas valve or PCB on a boiler that is already eight or more years old, it is worth getting a new boiler quote at the same time. Also check whether your boiler is still within its manufacturer's warranty — Ariston offers warranties ranging from two to twelve years depending on the model and whether the boiler has been annually serviced, and a valid warranty could mean the repair is covered at no cost to you.