Gas Safe registered
Ariston specialists
Clas ONE, E-Combi, Genus and the older Clas HE. Twenty years on Ariston boilers, rates agreed beforehand, and parts guaranteed for a year.
Answered 24 hours a day by a technical customer service member.
A P in the middle of an Ariston code means a warning rather than a lockout, so 1P1 and 103 are not the same thing.
No heating or hot water right now? Call 0203 868 6134. We will tell you the earliest realistic slot rather than the one you want to hear.
Two details to start. Add the fault code and your postcode and we can usually tell you the likely part before we attend.
Three steps, no sales pitch, and you will know where you stand before an engineer is booked.
Ariston is the one make that warns you before it breaks down, and almost nobody acts on it. The P codes are early flags rather than faults, raised while the boiler is still working, and dealing with one costs a fraction of what dealing with the lockout it turns into costs. If yours is showing a P code and you still have heat, that is the cheapest phone call you will make all winter.
Ariston faults cluster heavily on the water side, which is why so many of their codes are in the 100 group. Six parts cover most of what we attend.
Ariston codes look like arbitrary three-character strings and they are not. The first digit names the part of the system at fault, and a P in the middle means the boiler is warning you rather than stopping. Between them, those two rules tell you most of what you need before you look anything up.
What the boiler is saying: system pressure has dropped too low to run safely, so it stopped and is asking to be filled. The most common Ariston code and the only one most householders can clear themselves.
What you can safely do: switch the boiler off, let it cool, then open the filling loop until the gauge reads between 1 and 1.5 bar cold. Close it fully. Reset once.
What it is rarely: a boiler fault. The boiler is reporting the water in your pipework.
The exception: if the gauge reads a healthy 1.2 bar and 108 is still showing, the pressure sensor has failed rather than the pressure. Small part, short job.
What does the gauge read? That single number decides which of these it is.
The boiler ran its ignition sequence and never confirmed a flame, so it shut the gas off. The most common Ariston lockout.
Check first: does anything else in the property run on gas? A hob that will not light points at the supply rather than the boiler. Check a prepayment meter for credit while you are there.
In winter: the condensate pipe. That white plastic pipe running outside freezes, the boiler cannot discharge and it will not light. Pour warm water along it, never boiling, which splits it.
What it usually is the rest of the year: the ignition electrode or the sensing electrode, worn or carboned, then the gas valve, then a partly blocked flue.
The related code: 502 is the opposite and more serious, a flame detected while the gas valve is closed. That one is not for resetting.
Check the hob first. Ten seconds, and it changes the answer.
The water went past the temperature the safety system allows and the burner shut down. A 100 series code, so the fault is in the heating circuit rather than in the burner.
What we normally find: circulation. A jammed pump, a seized valve, sludge or trapped air, all of which leave the boiler making heat faster than the system carries it away so it cooks in one place.
What it is rarely: the heat exchanger, which is what the word overheat makes people fear. It is the last thing to suspect rather than the first.
Read it alongside the circulation codes. If you have had 1P1 to 1P3 warnings, or a 103 to 107 lockout, this is the same underlying problem arriving from a different direction.
What helps most: not a part. Getting the system properly clean deals with the cause where a component swap usually does not.
Leave it off and tell us whether any radiator was cold before this started.
Five codes describing the same underlying problem at different stages: the boiler is firing but the water is not moving through it properly, so it has stopped.
What we normally find: a pump that is running but not shifting anything, a closed or seized valve somewhere on the system, air, or a heat exchanger silted enough to restrict the flow.
Worth checking yourself: are all the radiator valves actually open? A thermostatic valve that has seized shut over the summer is a genuinely common cause and it costs nothing to find.
What you almost certainly saw first: a 1P code. These lockouts are what the P warnings escalate into, usually over weeks. If you remember seeing one, say so, because it dates the fault.
Which radiators get warm and which stay cold? That map is the diagnosis.
1P1, 1P2, 1P3 and on some models 1P4 are insufficient circulation warnings. The boiler is still running. It is telling you that flow is poor and that it is heading for a shutdown if nothing changes.
Why this is worth acting on immediately: almost no other make gives you this. Most boilers work perfectly until the moment they do not. An Ariston flags the problem while you still have heating, which means the repair can be booked at a convenient time rather than on the coldest night of the year.
What is behind it: the same three things behind a 103 lockout. Sludge, air, or a pump beginning to fail. All are cheaper to deal with before the boiler stops than after.
What not to do: ignore it because the heating still works. That is exactly what the warning is designed to prevent, and the lockout it becomes usually arrives in the middle of a cold snap.
The other P codes: 5P1 and 5P2 relate to ignition, and 3P9 is not a fault at all. See below.
Still got heat and a P code showing? That is the call worth making today.
3P9 is a service reminder. On the newer ONE series boilers it counts operating hours or time since the last service and puts this on the display when the interval is up.
What it means: the boiler is working and is due its annual check. Nothing has failed.
What people do instead: pay for an emergency call-out to be told their boiler is in perfect health. It is one of the more expensive misunderstandings on this brand.
What to do: book a service rather than a repair. The reminder usually clears as part of it, and if it does not, an engineer resets the counter.
Do not ignore it either. An unserviced Ariston is where the 1P warnings and the 100 group lockouts come from a couple of winters later.
Everything still working? Then this is a service, not an emergency.
If your display shows something like A01, A77, E02, E04 or E34, you have an older generation and none of the numeric codes above apply to it.
Why it matters: the two systems are not versions of each other. Looking up a modern Ariston code list with an A or E code in hand produces an answer that fits somebody else's boiler.
What to do: tell us the code and the range name together. On these generations the model is not optional information, it is half the diagnosis.
What it usually means for the repair: the boiler is old enough that availability rather than cost starts to decide things. That is worth establishing before anybody charges to attend.
Read us the code and the range name off the label together.
| Code | What the boiler is saying | What it usually turns out to be |
|---|---|---|
| 1 group | Central heating circuit | Pressure, circulation or a flow sensor |
| 2 group | Hot water and solar sensors | 201 to 204 are DHW sensors, not the boiler |
| 3 group | Internal electronics | The board, or a control talking to it |
| 304 | Reset more than five times in fifteen minutes | Not a new fault. Whatever you were resetting |
| 5 group | Flame and ignition | Electrodes first, gas valve second |
| 502 | Flame detected with the gas valve shut | Moisture on the electrode, or a valve passing |
| 6 group | Fan and flue | 607 and 612 are fan speed, check the flue too |
| Any code with a P | A warning, not a lockout | The boiler is still running. Act while it is |
The first digit of an Ariston code tells you which part of the system is at fault before you look anything up. A 1 is the central heating circuit, a 2 is the hot water side or the solar input, a 3 is the electronics, a 5 is the flame, and a 6 is the fan and flue. So a 108 and a 103 are both heating circuit problems, while a 501 is a completely different half of the appliance. That single digit narrows the conversation more than most people realise.
Ariston sell more than boilers, and several of the calls we take on this brand turn out not to be about the boiler at all.
Ariston is one of the few boiler manufacturers whose fault code list has a dedicated group for solar thermal, because they sell the panels and cylinders as well as the boiler.
What that means: a 200 series code on an Ariston in a property with solar panels may be reporting something on the roof or at the cylinder rather than inside the appliance on the wall. The sensor that has failed can be forty feet away from the boiler showing the code.
Why it catches people out: the boiler is the thing with a display, so it gets the blame. An engineer who does not ask about solar can spend a visit inside an appliance that is working perfectly.
What to tell us: whether there are panels, whether there is a cylinder, and roughly when the solar side was last looked at. Solar thermal is frequently the part of a system nobody has serviced in a decade.
Panels on the roof? Say so when you call. Call 0203 868 6134.
Nothing on the display, no lockout, and showers that are slightly worse each winter than the last. This is scale in the plate heat exchanger and it is the most gradual fault on any combi.
Why the heating stays faultless: that circuit is sealed, so the same water goes round, scales once and stops. The hot water side takes fresh mains water every time a tap opens, which in London means fresh scale every time.
Why people leave it too long: because it never breaks. There is no moment where you have to act, so the boiler quietly gets worse until somebody compares it to a neighbour's.
The window that matters: caught early enough it is a flush. Left long enough it is a replacement part. An annual service is what catches it in the first state rather than the second.
Ariston have renamed their ranges more often than most, and the names overlap in ways that make searching for your own boiler unusually difficult.
Clas, Clas HE, Clas ONE, Clas Net ONE, Genus, Genus ONE, E-Combi, E-System, Alteas ONE and Cares ONE are not ten different boilers. Several are the same platform under successive names, and some codes apply to only one or two of them.
What goes wrong: somebody looks up a code for a Clas ONE with a Clas HE on the wall, gets a plausible answer, and orders the wrong part.
What to do: read us the whole label, not just the range name. The serial number is what identifies which platform and which fault table yours belongs to.
Not sure which yours is? Send us the label. Call 0203 868 6134.
Cold radiators at the bottom or at the far end of the house mean magnetite has settled where the flow is slowest. No boiler part is involved and no boiler fault code names it.
On an Ariston it does get named eventually. This is the condition that produces a 1P warning, and then a 103 to 107 lockout, and then a 101 overheat. The boiler is watching it happen and telling you in stages.
Why it is worth catching at the radiator stage: cold radiators are a system clean. The same problem at the lockout stage often takes a pump with it.
What to check: feel each radiator top and bottom while the heating runs. Cold at the bottom is sludge, cold at the top is air, and the two need different work.
It warns you before it breaks down
A P in the middle of an Ariston code is a warning rather than a lockout, raised while the boiler is still running. Almost no other make gives you that notice. If yours is showing one and you still have heat, this is a cheap call.
The label is behind the front panel. On this make the range name alone is not enough, because several ranges share names and differ underneath.
Ariston name the range, then the type, then the output. A Clas ONE 30 is a Clas ONE rated 30 kilowatts.
If yours is a System, there is stored hot water somewhere in the property and the cylinder is as likely to be the fault as the boiler is.
Clas ONE, Clas Net ONE, Genus ONE, Alteas ONE and Cares ONE are the current generation. Three character numeric codes, the P warnings, and the 3P9 service reminder.
Clas HE, Genus and E-Combi are the previous generation. Mostly the same numeric system, though some codes apply to only part of the range.
Anything showing A or E followed by digits is older again and uses a different code family entirely. A general Ariston list will mislead you on those.
The practical test without a label: if your code has three characters and one of them may be a P, you are on the modern side. If it starts with a letter, you are not.
This matters more on Ariston than on most makes, because the names genuinely overlap. A Clas and a Clas ONE are not the same boiler, and a code that means one thing on one can mean something else on the other.
The serial number and GC number on the label identify exactly which platform is on your wall, and they are what gets an engineer to the correct fault table rather than a general one.
Read us the serial when you ring, not just the name. It takes ten seconds and on this brand it is the difference between the right part arriving and a second visit. That advice applies to every boiler brand we repair, without exception.
London is a hard water city almost everywhere, and that is the single biggest difference between repairing an Ariston here and repairing one in Manchester or Glasgow.
Chalk is the reason. The Thames and the Lea both run through it in the Chilterns and the North Downs before the water reaches your tap, and the result is roughly 277 milligrams of calcium carbonate per litre, ranging from about 250 to over 350 depending on the district. Inside a combi that means one thing: the plate heat exchanger furs up faster than the manual's service intervals allow for.
It is the same story in Highgate, Muswell Hill and Crouch End as it is in Dulwich. The water is hard, the plate heat exchanger scales, and the only question is whether anybody checks it before condemning the boiler.
Hardness is not uniform, and it does not follow borough boundaries. It varies by supply zone and can change over a short distance, so your supplier publishes a figure for your own postcode and it is worth reading before anybody guesses.
Scale drops out of solution faster the hotter the water gets. A combi set to deliver taps at 65 degrees furs up its plate heat exchanger noticeably faster than one set to 50, which is still hot enough to scald. Turning that dial down costs nothing and is the most effective thing a householder in a hard water area can do. This applies to the tap setting, not to a cylinder, which must reach 60 degrees for safety.
Stored hot water is normal in larger London houses, and no combi replaces it whatever the output figure says.
A great many London firms will only touch combis, which is awkward in a city with this much older housing stock. A Ariston system or heat only boiler feeds a separate cylinder carrying its own thermostat, immersion element and, where it is unvented, its own safety devices. Two appliances rather than one, which is why no hot water so often turns out to have nothing to do with the boiler at all.
The cylinder sensor lives in a pocket on the tank. If it fails or works loose the boiler stops heating the cylinder while every radiator stays warm, which is the single clearest sign the fault is not in the appliance.
The motorised valve sends the heat to the radiators, the cylinder or both. When its motor or microswitch goes you get heating with no hot water, hot water with no heating, or both at once, and the head usually changes without draining anything.
The immersion element furs up in London water and then either trips out or stops working. Test it before condemning a tank, because the element is a fraction of the cost. In larger houses across Sutton and Cheam we generally fit a scale reducer on the same visit.
Because an unvented cylinder holds mains pressure, it carries a set of safety devices: an expansion vessel, a pressure reducing valve, an expansion relief valve and a temperature and pressure relief valve, all discharging through a tundish. Water at the tundish is the system reporting that one has failed, and capping it is never the answer.
That work is notifiable and requires G3 as well as Gas Safe registration. We hold both, so the appliance and the cylinder are diagnosed on the same visit rather than across two. It is why our unvented cylinder repair is not farmed out to somebody else.
Removing a cylinder to reclaim an airing cupboard is the decision we are asked to undo more than any other. However large the output figure, a combi heats water as it passes rather than storing it, so it cannot serve two showers at once. With three bathrooms the right answer is almost always to keep stored hot water, and to replace an ageing cylinder rather than lose it.
The year it was fitted decides less than people think. What has actually failed decides almost everything, and we will say so either way.
| What has failed | Under 8 years | 8 to 14 years | 15 years and older |
|---|---|---|---|
| Pressure sensor, flow and return sensors | Repair | Repair | Repair if parts exist |
| Ignition and sensing electrodes | Repair | Repair | Repair if parts exist |
| Expansion vessel, pump | Repair | Repair | Repair |
| Fan | Repair | Repair | Depends on the rest of the boiler |
| Diverter valve, plate heat exchanger | Repair | Repair | Repair if the part is still made |
| Gas valve | Repair | Depends on the rest of the boiler | Replace |
| Control board | Check the guarantee first | Borderline, we will be straight with you | Replace |
| Main heat exchanger | Guarantee claim if registered | Usually replace | Replace |
Every part is current and at this age almost every failure is a single component rather than a boiler wearing out. An electrode, sensor, pump or fan on a six year old Clas ONE is a repair, without qualification.
Ask about guarantee cover when you ring, ourselves included. Ariston registered guarantees run for a set term from installation and are usually conditional on annual servicing, which is one more reason not to ignore a 3P9.
If a replacement has been suggested at this age, ask for the name of the failed part and its price on its own. The gap between those two answers is usually the whole conversation.
Still a clear repair: electrodes, sensors, pump, fan, expansion vessel, diverter valve. Ordinary wear items that say nothing about the condition of the rest of the boiler.
The conversation changes with a main heat exchanger or a genuine board failure, or with a second significant failure inside a year. At that point the arithmetic favours replacing the boiler, and the question is what goes in the next two winters as much as what has gone today.
The point specific to this brand: an Ariston in this band that has been showing P warnings and having them ignored is a different proposition from one that has not. Circulation problems left running take pumps and heat exchangers with them, and the history matters as much as the age.
Past this point availability takes over from cost. The older Ariston generations, the ones showing A or E codes, are where parts start to decide the outcome rather than price.
A boiler of that age can be mechanically sound and still finished because the one part it needs is no longer made, and a repair then is not slow or expensive, it is impossible.
We check availability before you pay for anybody to attend and tell you what has failed. That question is free and on an older Ariston it is the one worth asking first. See our guide to repair and replacement costs for what the number is made of.
Local boiler repair specialists
Clear advice. Careful repairs. Family-friendly service.
Call 0203 868 6134London, north and south of the river. These are the districts we work in most.
These are where we work most, not a boundary. An engineer attends anywhere in north London and south London. Tell us where you are and we will say plainly when we can get to you.
If you smell gas: do not touch any switches. Open the windows, leave the property, then call the National Gas Emergency Service on 0800 111 999. That line is free and open 24 hours. Call them before you call us.
If a carbon monoxide alarm is sounding: get everyone out immediately, then call 999 if anyone feels unwell. Do not go back in until the property has been ventilated and checked.
If the boiler locks out repeatedly: try one reset. If it locks out again straight away, stop. Repeated resets hide a fault that is still there, and on a gas appliance that is not a risk worth taking.
Under the Gas Safety (Installation and Use) Regulations 1998, only a Gas Safe registered engineer may work on gas appliances. Never open the boiler casing yourself.
Usually, yes. 108 means there is not enough water in the heating system for the boiler to run safely. Switch it off, let it cool, then open the filling loop until the gauge reads between 1 and 1.5 bar cold, close it fully and reset once. What you cannot fix is why it dropped: once a year is normal, every few weeks means a leak, every few days means the expansion vessel has lost its charge. If the gauge already reads 1.2 bar and 108 is still showing, the pressure sensor has failed rather than the pressure.
A P in the middle means a warning rather than a lockout, and it is the most useful thing this brand does. Codes like 1P1, 1P2 and 1P3 are the boiler flagging poor circulation while it is still running, before it becomes a 103 to 107 shutdown. Almost no other make gives you that notice. Deal with it while you still have heating and it is usually sludge, air or a pump starting to fail, all of which cost far less to fix now than after the boiler stops.
No. 3P9 is a service reminder on the newer ONE series boilers, counting time or operating hours since the last service. Nothing has failed and the boiler carries on working. People pay for emergency call-outs over this one, which is an expensive way to be told the appliance is healthy. Book a service rather than a repair. Do not ignore it either, because an unserviced Ariston is where the circulation warnings come from a couple of winters later.
501 means no flame was detected, so the boiler shut the gas off after trying to light. Check whether anything else in the property runs on gas first, because a hob that will not light points at the supply rather than the boiler, and check a prepayment meter for credit. In cold weather check the white condensate pipe outside for ice and thaw it with warm water, never boiling. Otherwise it is usually the electrodes, then the gas valve.
The first digit tells you before you look anything up. A 1 is the central heating circuit, so pressure, circulation or a flow sensor. A 2 is the hot water side or the solar input. A 3 is the electronics. A 5 is the flame and ignition. A 6 is the fan and flue. That means a 108 and a 103 are both heating circuit problems while a 501 is a different half of the appliance entirely, and knowing which half narrows the conversation considerably.
Yes, and it is worth saying so when you call. Ariston sell solar thermal as well as boilers, and their fault code list has a dedicated group for it, so a 200 series code on a property with panels may be reporting a sensor at the cylinder or on the roof rather than anything inside the boiler. The boiler gets blamed because it is the part with a display. Solar thermal is also frequently the section of a system nobody has serviced in a decade.
The label is behind the front panel, and on this make you want the whole label rather than just the range name. Ariston have used overlapping names across generations, so a Clas, a Clas HE and a Clas ONE are not the same boiler and a code can mean different things across them. ONE marks the recent generation, Net means app connected, and System means there is a cylinder somewhere. The serial number is what identifies which fault table yours belongs to. We work on every make of boiler.
No honest figure exists until somebody knows which part has gone. A price quoted before anybody has asked you a question is a number, not a quote. Three things decide the figure: the part itself, whether we are carrying it or ordering it, and the time on site. On an Ariston a P warning dealt with early is almost always cheaper than the lockout it turns into. The rate is agreed beforehand and parts we fit are guaranteed for a year, so nothing is carried out on the assumption that you will approve it afterwards. There is no charge for telling you a repair is not the right call. See our guide to repair and replacement costs for what the number is made of.
Ariston is one of the few makes whose fault codes are grouped by subsystem rather than numbered in sequence, which makes them easier to read once you know the rule.
No gas supply? Plenty of London flats run without one, and we also cover electric boiler repair.
If your Ariston is showing a P code and still working, that is the call worth making today rather than in January. Read us the code including any letter in the middle, and the range name from the label.
Gas Safe registered. Unvented cylinder qualified. Parts guaranteed for a year. Covering London, north and south of the river, 24 hours a day.
