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Common Ariston Boiler Problems


For more than half a century, Ariston has been providing efficient and high-quality heating solutions to customers in more than 150 countries worldwide. The company prides itself on offering innovative heating solutions at competitive and affordable price ranges.

If your Ariston boiler has been acting up with weird noises and constant lockouts, you are feeling frustrated. But before you call a Gas Safe engineer, here is a guide of common boiler faults and details on whether you can fix it yourself or you should hire a professional.

PCB fault

This is by far the most feared boiler problem. The PCB is the heart of a boiler. It sends are receives signals from other boiler components. A PCB fault cripples all boiler operations. And unfortunately, it’s the most expensive part to replace.

If your Ariston boiler displays fault codes 301 to 307, you probably have a PCB error. However, fault codes 303, 305, 307, and 306 are the most challenging to fix. And since they are the main PCB faults, there’s a high possibility you will replace the unit.


Call a Gas Safe engineer to inspect and assess the damage. It might be weak or damaged PCB connections that can be repaired inexpensively.

If you need to replace the PCB, prepare yourself to spend £500+. And if the boiler is old and out of warranty, consider installing a new boiler instead.

Low water pressure

Boiler pressure drops with time. However, there’s a perfect boiler pressure range between 1 bar and 1.5 bar. Usually, this range is marked green on the pressure gauge. If the boiler pressure drops below 0.6 bar, your Ariston boiler will display a 108 fault code.

Though low boiler pressure is no cause for alarm, you should have it fixed soon to avoid straining and damaging other boiler components. Luckily, you can fix this fault alone.


Boiler pressure is directly related to water levels in the boiler. Adding water to the system through the external filling loop will increase the boiler pressure to the normal range. After a while, you should reset the boiler to clear the boiler fault.

Boiler leaks

If you receive the 108 boiler fault soon after clearing it, it means you have a leak in the system. Boiler leaks can occur in any part of the boiler. The simplest to fix are those on the external piping. These leaks are often accompanied by an unexplained pool of waters in the house.

Leaks on piping in the walls and inside the boiler are difficult to fix because of the tools and expertise required. The leaks can be caused by joints loosened by pressure over time or cracks in the boiler pump.


Call a Gas Safe engineer to inspect the boiler for leaks. For leaks on the joints, they will tighten or seal the leak. However, if a crack on the boiler pump causes the leak, it will need replacement.

New boiler replacements cost about £300.

Ignition Failure

If you have an ignition fault, you’ll see a 5P2 or a 5P1 displayed on the boiler. If the boiler doesn’t ignite, it could be because of:

  • Faulty ignition lead
  • A blocked burner
  • Faulty spark generator
  • Blocked pilot jet


Most of the faults outlined need minimal repairs for the boiler to ignite. However, if a boiler component is blocked and is in bad condition, you will have to replace it. If the boiler is old and out of warranty, you should calculate the cost of replacement against the cost of repair.

When a boiler is old, its parts start to deteriorate and malfunction. You shouldn’t be stuck in an endless repair cycle spending a lot of cash on an old boiler – install a new boiler.

No flame

The pilot light stays on ready to light the boiler burner when you need hot water or turn on central heating. Without the pilot light, the Ariston boiler will not heat water or turn on. When it’s on, the pilot light should be blue. If it’s orange or yellow, then there might be a carbon monoxide leak.

Carbon monoxide is a deadly gas mainly because it is odourless and colourless. You cannot detect it without a carbon dioxide alarm. As such, Gas Safe engineers insist on its installation when installing a new boiler.

If the pilot light turns orange or yellow, you should open all windows and contact a Gas Safe engineer ASAP.

The things that could prevent a boiler flame include:

  • A faulty thermocouple
  • A block in the flue
  • Blocked gas valve
  • Strong draft putting out the flame


You should not attempt to fix the 501 fault code yourself. Call a Gas Safe engineer. The engineer will dismantle the boiler, clean the pilot jet, and put it back.

Also, they’ll check for other possible causes and fix them.

Fan fault

The fan is an important boiler component. However, unlike in other electrical appliances, the fan is not tasked with cooling the boiler. It’s installed to push harmful gases into the flue for expulsion. When you turn on your Ariston boiler, it’s the first component that starts. If it’s faulty, the boiler will not start and display an A41 fault code.


An Ariston boiler will not start if the fan is faulty. The fan needs to be fixed by a Gas Safe engineer. The possible fan faults include:

  • Incorrect fan settings – a fan might be running too fast or too slow (fault code 604), causing the flame to go out or not creating enough draught to push harmful gases into the flue, respectively. In this case, the Gas Safe engineer will correct the settings and attempt to restart the boiler.
  • Broken fan – the fan needs replacement. But if it’s older than eight years, it would be cheaper installing a new Ariston boiler.

Insufficient circulation

If the boiler shows any fault code between 103 and 108, your boiler has insufficient circulation. This could be caused by:

  • A block in the system
  • Problem with the heating pump

If the heating pump isn’t working efficiently, the radiators and your hot water will not get to your desired temperatures. The causative agents could include:

  • Airlock in the heating pump
  • Worn out pump shaft or bearings
  • Blown pump seal
  • Incorrect speeds
  • Incorrect installation

Boiler blocks are often caused by sludge build-up. Sludge is made up of rust and grime in the boiler and radiator. A block in the boiler means water circulation will be impaired, and pressure will increase.


If you suspect you are dealing with a faulty pump, hire a Gas Safe engineer to replace it. If it’s a block, have the Gas Safe engineer conduct a Powerflush or a Chemical Flush to rid the system of the sludge.

The engineer will also recommend installing a magnetic filter to prevent future sludge build-up. But if the block damaged the heating pump, you will need to replace it.

Air pressure switch fault

The new Ariston boilers have fault codes different from the old Ariston boilers. For instance, in the Microgenus, an E34 code will be displayed in case of an air pressure switch failure.

The boiler air pressure switch makes sure the fan pushes harmful boiler gases into the flue for expulsion. In case of a fault with this boiler component, the boiler isn’t safe for operation. This is because the PCB cannot tell if the boiler is expelling the harmful gases or not.


If you are in luck, this fault will be caused by an air pressure switch fault only. Unfortunately, these codes aren’t completely accurate. Other problems could be the connection between the switch and the PCB or a faulty PCB.

If the PCB needs replacing, it’s better to install a new boiler instead.

High boiler pressure

Low boiler pressure isn’t good, and so is high boiler pressure. When in operation, the boiler pressure gauge might read 2 bar, which is above the normal. However, this is acceptable because of the pumping pressure. When not in use, the pressure should reduce to normal.

However, if the boiler pressure rises to 3 bar, it is cause for alarm. The common causes for high boiler pressure include:

  • A lot of water in the system. This is often caused by excessive water added during pressurization. When adding water, always have your eye on the pressure gauge.
  • Malfunctioning boiler parts, including the filling loop and pressure release valve.


For excess water, you should drain the system through the radiators. Purchase a radiator bleed key from a local DIY store and use it to turn the radiator valve. For faulty filing loops, you’ll need to replace the defective parts.

A new filling loop costs between £80 and £150 while a pressure release valve costs between £100 and £350. These fixes should clear the 109 fault code.

Boiler overheating

An overheating boiler is an emergency and displayed by a 722, 723, or 101 fault code. You should never attempt to fix an overheating boiler yourself. This problem is caused by failing boiler safety control components including those that regulate temperature, water, and gas supply.

If not fixed immediately, the boiler components are stressed and can sustain irreparable damage. In some instances, the boiler can explode.

Other malfunctions related to boiler overheating include:

  • Wiring faults
  • Manually opening the boiler gas valves
  • Removing pressure relief valves


Call a Gas Safe engineer to replace the pressure release valves and fix wiring faults. Also, reduce boiler pressure to normal range.

Other boiler fault codes and troubleshooting

Fault code 712: Defective return sensor in zone 2

How to fix: Replace the return sensor.

Fault code 5P3: Flame cut of

How to fix: Reduce fan speed.

Fault code 3P9: Schedule boiler service

How to fix: N/A

Fault code 304: Too many rests within 15 minutes

How to fix: Find the cause of boiler lockout.

How to improve your boiler’s life

Ariston boiler guarantee

The standard Ariston boiler warranty is two years. However, some Ariston boilers like the Class HE System Evo and the Clas HE Evo have between 8 and 12-year warranties.

During the warranty period, Ariston will replace and replace faulty boiler components as a result of issues with workmanship or materials used. The warranties don’t cover problems arising from maintenance and commissioning procedures and incorrect installation.

To validate the warranty for each year, you need to conduct an annual boiler service and present the resulting certificate.

Ariston boiler cover

When your boiler warranty comes to an end, you should purchase a boiler cover to protect you from sudden and high boiler repair and replacement costs. However, these boiler covers require you to pay monthly premiums.

You can take a cover with Ariston boilers or a different boiler insurance company provided they have favourable terms. Always ensure they cover annual service and safety inspections.

Ariston boiler service

Boiler servicing should be done annually and by a Gas Safe boiler engineer. The servicing shouldn’t last more than 30 minutes. However, the time taken depends on the boiler type and the number of faults to be fixed.

An annual Ariston boiler service is essential for:

  • Safety
  • Keeping the heating bills at a minimum through increased boiler efficiency
  • Preventing frequent and sudden breakdowns
  • Maintaining the boiler warranty

Remember, only use Gas Safe engineers to fix boiler faults. If you need repair or new boiler installation quotations from registered boiler engineers, reach out to us. We shall provide three free quotes from certified engineers in our database.

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