Ariston 5P1 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 5P1 fault code mean?
The 5P1 code appears on Ariston boilers when the burner fails to ignite on the first attempt. Because Ariston's seven-segment display renders the digit '5' in a way that closely resembles an 'S', many homeowners read this as 'SP1' — they are the same fault. The boiler is telling you it tried to light the burner and couldn't. Ariston uses a progression of these codes: 5P1 for the first failed attempt, 5P2 for a second failed attempt, and 5P3 for a third consecutive failure, at which point the boiler locks out completely and requires a manual reset before it will try again. Older Ariston models may display SP1 or SP2 rather than the newer 5P1 notation, but the underlying fault is identical. This is an ignition lockout notification rather than a sign of catastrophic failure — in many cases the cause is straightforward — but repeated occurrences always warrant investigation by a Gas Safe registered engineer.
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 low gas supply Common
If the gas meter isolation valve has been turned off, a neighbouring supply interruption has occurred, or there is low pressure on the network, the boiler cannot receive enough gas to ignite the burner. This is the simplest cause to rule out yourself — check whether other gas appliances such as a hob or gas fire are working normally.
- Faulty or contaminated ignition electrode Common
The spark electrode generates the ignition spark and must sit at a precise gap from the burner. Over time the electrode tip can become coated in carbon deposits, crack, or shift out of position, preventing a reliable spark. This is one of the most frequently replaced parts in an ignition fault diagnosis.
- Frozen or blocked condensate pipe Common
In cold weather, the condensate pipe that runs outside the property can freeze solid, causing the boiler to shut down. Because the blockage stops the boiler from operating entirely, the first error it logs is often an ignition failure. This is especially common after overnight frosts.
- Faulty ignition lead or spark generator Sometimes
The high-voltage lead that carries the spark signal from the PCB to the electrode can deteriorate, crack, or lose its connection over time. A damaged lead produces a weak or absent spark, preventing ignition even when gas is present and the electrode is in good condition.
- Gas valve not opening correctly Sometimes
The gas valve is a solenoid-operated component that releases gas to the burner on command from the PCB. If the valve is sticking, has a faulty solenoid coil, or has suffered internal wear, gas may not reach the burner at the moment the spark fires — leading directly to a 5P1 fault. This requires Gas Safe engineer diagnosis and, if confirmed, valve replacement.
- Dirty or partially blocked burner Sometimes
Debris, dust, or corrosion products can accumulate on the burner ports over time, particularly in boilers over seven years old. A partially blocked burner restricts gas flow to the point where ignition either fails entirely or produces an unstable flame that does not register with the flame sensor.
- PCB or control circuit fault Rare
The printed circuit board orchestrates the entire ignition sequence — it triggers the spark, opens the gas valve, and monitors the flame sensor. An internal PCB fault, a loose wiring harness connector, or moisture ingress can disrupt this sequence and cause a spurious 5P1. PCB faults are usually a diagnosis of exclusion after other components have been checked.
How to fix it
- Check that your gas supply is working DIY safe
Before doing anything else, try another gas appliance in your home — a gas hob ring or gas fire. If none of your gas appliances are working, the problem is almost certainly with your gas supply rather than the boiler. Contact your gas network provider (the number is on your bill or on the national grid website) rather than calling a boiler engineer.
- Top up boiler pressure if it is below 1 bar DIY safe
Find the pressure gauge on your Ariston boiler — it is usually a dial or a digital reading on the display. If the reading is below 1.0 bar when the system is cold, the boiler may fail to attempt ignition safely. Use the filling loop (a braided flexible hose beneath the boiler with one or two valves) to slowly admit mains water until the gauge reads around 1.2–1.5 bar, then close the filling loop valves. Do not overfill past 2.0 bar.
- Thaw a frozen condensate pipe if it is cold outside DIY safe
The condensate pipe is a white or grey plastic pipe (usually 21.5 mm or 32 mm diameter) that exits through an outside wall or runs into an external drain. If the outside temperature has been at or below freezing, gently pour warm — not boiling — water along the length of the pipe until it thaws. Once the pipe is clear, attempt a single boiler reset. After a successful restart, insulate the pipe with lagging to reduce the chance of refreezing.
- Reset the boiler once or twice DIY safe
Press and hold the reset button (consult your Ariston user guide for the exact location on your model — it is commonly a button marked with a flame and a cross, or labelled 'Reset') for around five seconds until the display clears, then release it and allow the boiler to run through its startup sequence. If the 5P1 code returns immediately or within a short time, a single further reset is reasonable, but do not keep resetting repeatedly — doing so can stress electrical components and may mask a worsening underlying fault.
- Do not attempt to access internal gas components Gas Safe engineer
Work on the gas valve, ignition electrodes, ignition leads, burner assembly, or PCB must only be carried out by a Gas Safe registered engineer. It is a legal requirement in the UK, and attempting this work yourself can create a serious risk of gas leak, explosion, or carbon monoxide poisoning. Do not remove the boiler casing for anything beyond the reset button.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the 5P1 code persists after the homeowner checks above, a Gas Safe engineer should visit to carry out a full ignition sequence test. They will check gas pressure at the burner, inspect and clean or replace the electrode and ignition lead, test the gas valve solenoid, examine the flame sensor (ionisation probe), and assess the PCB if other components test satisfactory. Always ask to see the engineer's Gas Safe ID card before any work begins — you can also verify their registration at gassaferegister.co.uk.
Parts you may need
- Ignition electrode · from £35
- Ignition lead (HT lead) · from £25
- Spark generator module · from £55
- Gas valve · from £130
- Ionisation / flame sensing probe · from £30
- 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–£350, depending on the underlying cause.
Frequently asked questions
Is 5P1 the same as SP1 on an Ariston boiler?
Yes, they are exactly the same fault. Ariston's LED display renders the numeral '5' in a way that looks very similar to the letter 'S', so the code 5P1 is routinely misread as SP1. You may also see 5P2 (second failed ignition attempt) or 5P3 (third attempt, full lockout requiring a manual reset). All three relate to ignition failure and should be investigated in the same way.
Can I keep resetting the boiler to clear the 5P1 code?
You can try one or two resets, particularly after ruling out a frozen condensate pipe or a temporary gas supply interruption. However, repeatedly resetting a boiler that keeps returning the same fault is not a fix — it simply masks the underlying problem and can put additional strain on electrical components. If the code reappears after a couple of resets, stop resetting and call a Gas Safe engineer.
How much does it cost to fix a 5P1 fault on an Ariston boiler in the UK?
Most homeowners with a 5P1 fault end up paying somewhere between £150 and £350 including parts and labour. A straightforward electrode clean or replacement sits at the lower end of that range, while a gas valve replacement typically lands in the middle to upper end. If the PCB turns out to be the cause, replacement costs are higher — often £300–£500 — but this is a less common outcome. Always get a written quote before authorising work, and check whether your boiler is still within its manufacturer warranty (Ariston warranties range from 2 to 12 years depending on the model and registration).
My Ariston boiler keeps showing 5P1 every morning in winter — why?
A pattern of early-morning 5P1 faults in cold weather is a strong indicator that the external condensate pipe is freezing overnight. The pipe carries acidic condensate water away from the boiler, and when it freezes solid the boiler cannot operate. The immediate fix is to thaw the pipe with warm water and then reset the boiler. The lasting fix — which a heating engineer can help with — is to properly lag the exposed section of pipe with weather-resistant insulation, or to reroute it internally if the layout allows.