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Common ATAG Boiler Problems


ATAG has been manufacturing boilers for over 70 years. During this time, they have set high energy efficiency and engineering standard. They build their boilers using high-quality brass and stainless steel.

Picture this, you turn on your boiler, it turns on for a few seconds and then turns off. Or the heating never gets to the set temperatures. These problems are often accompanied by fault codes and light codes you cannot decode. But what is an inconvenience and feels like rocket science is quite simple.

We have broken down some of the boiler problems and matched them to their fault codes for easy diagnosis and fixing.

Boiler comes on without need

The digital wireless stats can lead the ATAG boiler to turn on without being asked for. This can lead to energy loss especially when you don’t need the hot water or heating.


When this occurs, inspect the minimum room temperature. It should be around 8 and 12 degrees Celsius. If it’s too high, then the digital room stat will signal the boiler to turn on and heat the room.

Low boiler pressure (fault code 118)

This error code means the boiler pressure is below 1 bar or the pump pressure is low. Overall, boiler pressure is expected to drop over time. However, a sudden pressure loss is a sign of a leak within or without the system.


Increasing boiler pressure by adding water to the system through the external filling loop should get rid of the fault code. However, if this doesn’t work, hire a Gas Safe engineer to inspect the boiler for internal and external leaks. Internal leaks are often caused by cracks in the pump seals while external leaks are often found on the piping.

No hot water or heating

Combi boilers provide both hot water and heating simultaneously. They switch between these functions with the help of a diverter valve. However other faults in the system could lead to the lack of heating or hot water. These faults include:

  • A faulty thermostat
  • Airlock in the system
  • Low water levels
  • Faulty motorized valves

If you find you either have heating and no hot water or have hot water and no heating, you should know the diverter valve is stuck.


Hire a Gas Safe engineer to free the diverter valve. However, if the diverter valve is faulty, it will need replacement. The price of a new diverter valve is about £250. And if a boiler is out of warranty, consider installing a new boiler.

Radiators not heating up

If the radiator is not heating, or only the bottom halves are hot, it could mean there is air trapped in the boiler. Air in the system helps to keep water flowing. However, too much air blocks the system like sludge and debris. The air prevents hot water from circulating to portions in the radiator.

To get the water flowing in the radiators, you should bleed the system.


Before you start bleeding your radiators, you should ensure the boiler is off. Also, allow the boiler to cool off to avoid hot water accidents. You can follow simple steps to bleed the radiator.

However, if you are unsure of your ability to pull it off, don’t hesitate to hire a Gas Safe engineer to bleed the system for you.

Ps: don’t forget to re-pressurise the system after bleeding it.

Boiler fan does not start

Unlike fans in other systems, boiler fans don’t cool the system. They create a draught that sends harmful boiler gases into the flue for expulsion. When you turn on the boiler, the fan first switches on and then the boiler kicks in a few seconds later.

If a fan doesn’t start, the boiler locks out and doesn’t start and displays a 129 fault code. This is a safety mechanism to keep you safe.


The possible causes of the above problem include:

  • A faulty fan – the fan cannot start
  • Loose wiring or connections – where the PCB doesn’t sense the fan has started

A Gas Safe engineer will use a multi-meter to inspect the wiring and connections. They will tighten and replace connections to restore communication between the PCB and the fan.

But if the fan is faulty, they will replace it for about £250 including installations.

Boiler overheating

Boiler overheating is serious and could result in an explosion. Fortunately, ATAG boilers are fitted with safety measures that lock out the boiler and display 101 fault code before the temperature gets to dangerous levels.

The common causes of boiler overheating include:

  • A block in the boiler system
  • Faulty pump
  • Limescale build-up
  • Faulty thermistor


Hire a Gas Safe engineer to diagnose the cause and solve it. For limescale and sludge build-up in the system, a Powerflush is necessary. However, they will replace a faulty thermistor and pump.

And to prevent future boiler overheating, a magnetic filter will be installed to trap limescale and sludge in the system.

Boiler pressure too high

Normal boiler pressure is between 1 bar and 1.5 bar. This pressure range is marked green on the pressure gauge. High boiler pressure causes the boiler to lockout and display a 117 fault code.

Common causes of high boiler pressure include:

  • System overfilling
  • Faulty pressure release unit
  • Faulty pump


You can bleed the system to relieve the pressure. Always keep an eye on the pressure gauge to guide you on when to stop the bleed. If the fault code doesn’t disappear, hire a Gas Safe engineer to repair or replace the pump.

Boiler not firing up

There are multiple reasons why an ATAG boiler fails to fire up and display fault code 133. The most common causes include:

  • A faulty fan
  • No power
  • No gas supply
  • Frozen condensate pipe


Since the problem is not specific, you’ll have to troubleshoot the system. First check the power is on and that there’s gas supply. If these are okay, the condensate pipe might be blocked.

A condensate pipe carries boiler waste water into the drainage system. Since it’s located outside, it’s prone to freezing and blockage during cold weather. When it is blocked, the boiler locks out. Luckily, you can thaw and unblock the pipe without a Gas Safe engineer.

No flame detected

If there’s no flame detection, you’ll see a 501 fault code. Though there are several potential causes, the most common is a blocked pilot jet. Unfortunately, it’s not easy to diagnose.

But if the pilot light is on but the 501 fault code is still displayed, the problem could be a faulty flame sensor.


The Gas Safe engineer will dismantle the boiler, clean the pilot jet and reinstall it. But more importantly, they’ll be looking for problems affecting boiler flame sensing and the ignition sequence.

Faulty PCB

The PCB is the heart of an ATAG boiler. When it fails a myriad of things go wrong in a system. Symptoms of a faulty PCB aside from the 303 fault code includes:

  • Intermittent boiler display operation
  • No power to the display panel
  • Unpredictable boiler operation

The causes of PCB faults include:

  • Leaking boiler
  • Damaged or weak joints
  • Fractures and cracks on the PCB


The Gas Safe engineer will repair or replace the PCB. A PCB replacement costs about £500. As such, a new boiler installation is preferred especially when the boiler is out of warranty.

Other fault codes and troubleshooting

Fault code 110 – Exceeded safety temperature.

How to fix: Replace the NTC thermistor

Fault code 612 – Faulty fan

How to fix: Replace the fan

Fault code 102 – Pressure sensor fault

How to fix: Clean carbon and debris build from the system

Fault code 010 – Too many boiler resets

How to fix: If the boiler keeps restarting, call a Gas Safe engineer for inspection and diagnosis

How to improve your boilers life

ATAG Boiler Warranty

ATAG boilers have a 10-year standard warranty. During the warranty period, ATAG sends their engineers to fix boiler faults. Generally, the longer the manufacturer warranty the more faith you should have in the product.

However, the warranty only covers some boiler parts and may not cover labour. Ensure you read the fine print on the warranty to know what is on offer.

ATAG Boiler Service

ATAG boilers should be serviced by a Gas Safe engineer. The annual service is necessary to maintain the boiler warranty. Moreover, it is essential in ensuring:

  • Safety
  • Catch the problems early
  • And keep the energy bills low with increased heating efficiency
  • ATAG Boiler Cover

When the warranty is expired, you can take boiler insurance as well. The insurance protects you from high and sudden repair or replacement costs. However, you are not obligated to take one.

Now that you know how to diagnose and fix boiler faults, it’s important to know who to go to for servicing and maintenance. Send an enquiry and we shall send several free quotes from trusted boiler engineers in the UK.

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