Ariston 5P4 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 5P4 fault code mean?
The 5P4 code appears when your Ariston boiler has attempted to ignite or hold a flame three times within a single working cycle and failed every time. After that third failure the boiler locks itself out and will not restart automatically. It sits at the end of a family of escalating codes — 5P1 (one failed attempt), 5P2 (two failed attempts), and 5P3 (flame cut-off) — with 5P4 representing the point at which the boiler gives up and shuts down for safety. Until the underlying cause is resolved and the boiler is reset, it will remain locked out.
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
- Interrupted or absent gas supply Common
If your gas meter has been turned off, a prepayment meter has run out of credit, or there is a wider supply disruption on your street, the boiler will fail to light every time. This is the simplest cause to rule out — try lighting a gas hob on the same supply.
- Faulty ignition electrode or ionisation sensor Common
The ignition electrode creates the spark that lights the burner, while the ionisation (flame sensing) electrode confirms a flame is present. If either is worn, cracked, incorrectly gapped, or coated in deposits, the boiler may fail to ignite or fail to detect a flame it has already lit, triggering repeated lockouts.
- Faulty or sticking gas valve Common
The gas valve controls exactly how much gas reaches the burner. A valve that is slow to open, sticking, or failing to modulate correctly can starve the burner of gas mid-cycle, causing the flame to drop out. This is a common reason for the 5P4 code persisting after electrode replacement.
- Low or fluctuating gas pressure at the boiler Sometimes
Even if your supply is on, the pressure arriving at the boiler may be insufficient. Low gas pressure can result from a partly closed service valve, a problem at the meter, a failing regulator, or high demand on the network. An engineer can measure the working gas pressure directly at the boiler.
- PCB fault Sometimes
The printed circuit board governs the ignition sequence, reads the flame signal, and controls the gas valve transistors. Cold solder joints, failing capacitors, or overheating components near the gas valve drive circuitry can all cause intermittent or repeated ignition failures. PCB faults tend to be suspected once gas-side components have been tested and found to be in order.
- Blocked burner or dirty combustion components Sometimes
A build-up of debris, scale, or carbon deposits on the burner or heat exchanger can disrupt the air-to-gas ratio needed for stable combustion, causing the flame to establish briefly and then cut out. This is more likely on boilers that have missed annual servicing.
How to fix it
- Check your gas supply is live DIY safe
Before anything else, confirm that your gas is turned on at the meter. If you have a prepayment meter, check you are in credit. Try a gas hob or another gas appliance — if they also fail to ignite, the issue is almost certainly with your supply rather than the boiler itself. Contact your gas supplier if there is no supply.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Ariston boiler for approximately five seconds until the display responds. Allow the boiler a full ignition attempt — this can take up to 30 seconds. If the 5P4 code clears and the boiler runs normally, monitor it over the next day or so. If the code returns immediately or after a short time, do not keep resetting — repeated resets without fixing the cause will not help and may mask a developing fault.
- Call a Gas Safe registered engineer to diagnose the fault Gas Safe engineer
Because 5P4 is a lockout triggered by three consecutive flame failures, an engineer needs to identify which component is at fault. They will check gas pressure at the boiler, inspect the ignition and ionisation electrodes, test the gas valve operation, and evaluate the PCB. Do not attempt to access the gas valve, electrodes, or PCB yourself — this work is legally restricted to Gas Safe registered engineers in the UK.
- Engineer to inspect and test the ignition and ionisation electrodes Gas Safe engineer
The engineer will check both electrodes for physical condition, correct positioning, and electrical continuity. Worn or fouled electrodes are among the most common causes of repeated flame failures on Ariston boilers and are relatively straightforward to replace.
- Engineer to test and, if necessary, replace the gas valve Gas Safe engineer
The engineer will measure gas pressure and test how the valve opens and closes during the ignition sequence. A valve that is sticking or failing to open fully will need replacing. Gas valve work must only be carried out by a Gas Safe registered engineer.
- Engineer to inspect the PCB if gas-side components are serviceable Gas Safe engineer
If the electrodes and gas valve are functioning correctly, the engineer will turn attention to the PCB. They will look for damaged components, cold solder joints, and any signs of overheating around the gas valve control circuitry. On older Ariston boilers, PCB repair or replacement can be costly — if the rest of the boiler is ageing, your engineer should discuss whether a new boiler represents better value.
Parts you may need
- Ignition electrode · from £25
- Ionisation / flame sensing electrode · from £22
- Gas valve (Ariston compatible) · from £95
- PCB (Ariston replacement) · from £180
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–£320, depending on the underlying cause.
Frequently asked questions
Can I keep resetting the boiler to clear a 5P4 fault?
A single reset is worth trying, but if the code comes back you should stop there. The 5P4 code means the boiler has already retried three times on its own — repeatedly resetting it without fixing the root cause will not help and could in rare cases stress components further. Once it locks out again after a reset, call a Gas Safe engineer.
Does the 5P4 code apply to all Ariston boiler models?
Yes — Ariston uses the same fault-code system across their range, so the 5P4 code and the 5xx ignition-fault family apply to most current and recent models including the Clas, Genus, E-Combi Evo, and Altus. If you are unsure which model you have, the model name and serial number are usually printed on a label inside the front casing.
My boiler is still under warranty — will Ariston cover a 5P4 fault?
Ariston boilers typically come with a manufacturer warranty of between two and twelve years depending on the model and registration. If your boiler is within its warranty period, contact Ariston or your installer before paying for repairs, as covered faults should be rectified at no charge. Keep your installation paperwork and service records to hand when you call.
What does it cost to fix a 5P4 fault, and what if the PCB is involved?
For the most common causes — ignition or ionisation electrodes and gas valve work — most homeowners pay somewhere between £120 and £320 all-in including parts, labour, and call-out. If the PCB is found to be faulty, replacement parts alone can run to £180–£250 or more, pushing the total to £400–£550 or beyond depending on the engineer's labour rate. On an older boiler, a PCB repair at that price can be difficult to justify — it is worth asking your engineer for an honest assessment of the boiler's overall condition and getting a new-boiler quote for comparison.