Greenstar 4000 Combi
If you're looking to diagnose the problem, the first port of call should be your boiler's manual. It's a font of knowledge that will explain fault codes and let you know if you can fix the problem yourself.
If you can't find your manual, most manufacturers will have an online copy available to download on their websites.
Read on find out the most common faults your boiler may develop, and how much you should expect to pay for common repairs.
The majority of serious boiler faults are hard for a layman to diagnose, and if you're not a trained heating professional you should never attempt any repairs that involve removing the boiler casing. It's dangerous to do so and also invalidates any warranty you may have remaining on the boiler.
However, two of the three most common boiler faults can be handled by a competent DIYer and don't always require the attention of a costly heating engineer.
When there's a cold snap, it's remarkably common for a frozen condensate pipe (a condensate pipe is a feature of condensing boilers) to put your boiler out of action. It's not technically a fault with the boiler but if the condensate pipe freezes, the boiler will shut itself down for safety reasons.
Before you incur a costly call-out charge from an engineer, always make sure you've checked that the condensate pipe isn't the problem. Always make sure that your condensate is properly insulated with foam, as in the image above, as this should prevent it from freezing in the first place.
You can repressurise your system yourself by opening the filling loop taps on your boiler until you achieve the desired pressure (usually around 1 bar). Once this is done, remember to close both taps, at which point your boiler should start to work again.
If your boiler is consistently losing pressure then you should have it looked at by a trained engineer to fix the issue for good.
While the three faults above are relatively common, there are lots of specific and technical faults that can occur with your boiler that a heating engineer will be able to help you with.
In 2021 we asked 153 how much they would charge for parts and labour to fix the 10 most common boiler faults. Make sure you cross reference any quotes you get for fixing your boiler against the prices above and make sure you're not ripped off.
In the table below you can learn more about the most common boiler parts that fail. Only Which? members can unlock the table and reveal what those parts affect and how much it typically costs to get them replaced. If you're not a member, now.
Boiler part name
Cost to replace part (including installation)
What this boiler part does
Directs flow of heated water from the boiler for heating or hot water
Printed circuit board
The ‘brain’ – it makes things run together
Moves heated water from the boiler, for either heating or hot water
Fan for combustion
Moves air through the boiler for combustion
Regulates gas flow to the burner
Prevents boiler from overheating
Automatic air vent
Lets air out of the boiler
Pressure relief valve
Safety component that ensures pressure does not become dangerous
Thermocouple (only found in older boilers)
Essential component in preventing gas leaks if your pilot light goes out
Ignites the pilot/main flame
To avoid any future problems with your boiler, it's worth stressing the importance of getting your system serviced annually. An annual boiler service is the best way to keep it in good working order, diagnose problems early and limit costly repairs.
Failure to get your boiler serviced annually will in most cases invalidate its warranty.
In our 2021 survey of 153 Which? Trusted Traders, the average price of a service was £86, and it's definitely worth doing to keep you safe and warm through the colder months.
CO is a colourless, odourless gas that is produced by incomplete burning of carbon-based fuels, including gas. CO stops the blood carrying oxygen around the body and can kill quickly.
Symptoms include headaches, breathlessness, nausea, dizziness, loss of consciousness, drowsiness, vomiting, pains in the chest, stomach pains or visual problems.
An annual service is also essential to make sure that your boiler remains safe.
Owners of old fashioned non-room sealed boilers and systems with hidden flues are particularly at risk from the dangers of carbon monoxide.
If after reading this guide you decide you need a heating engineer to diagnose and fix your boiler problem, make sure you use a . You can also use our Trusted Traders search tool below to find a local trader in your area. You don't have to be a Which? member to use Which? Trusted Traders.