The only Vaillant error code worse than 3, but better than 5. Welcome to F4.
As with most boiler brands, Viessmann boilers come with a variety of error codes.
These built-in codes are included as a means of alerting homeowners to specific problems should they arise.
Of course, by following the Viessmann manual, you’ll learn what various fault codes mean and their potential solutions.
But what about the Viessmann F4 fault code specifically? What does it mean and how do you fix it?
Let’s dive in!
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What Does the F4 Fault Code Mean?
An F4 fault occurs when your boiler’s flame cannot be detected.
Should this happen, along with the F4 error being displayed, your boiler will enter a lockout state. This is done for your safety and to protect your boiler.
The boiler flame is used to burn fuel before reaching the boiler unit.
As your boiler functions, it will send signals to your PCB which is pretty much the control centre of the boiler.
If the boiler does not receive a signal confirming that the boiler flame is working as intended, it will lock out and trigger an F4 fault.
What Causes No Flame to Be Detected?
There is a range of possible reasons for an F4 fault, including:
Gas valves that are stuck
Each gas valve is used to ensure the right amount of gas gets into your boiler. They are generally kept in the closed position.
However, when you want to turn the heating on, they will slide to the open position, allowing gas to enter.
Unfortunately, it’s possible for gas valves to become stuck, thus preventing your boiler flame from lighting due to a lack of fuel access.
Wiring/connection problems
It’s possible that a component such as your gas valves is not able to send accurate signals to the PCB because of faulty wiring or connections.
This would ultimately lead your PCB to trigger an F4 lockout. This could occur even if the flame is lit but the PCB cannot detect it.
The flame sensor is faulty or impaired
Aside from the actual cables and wiring, the flame sensor is the component (connected via cabling/wiring) that tells the PCB whether the flame is lit or not.
There are many ways this sensor could fail. For one, it’s possible that the porcelain insulation that the sensor is encased in could experience cracks.
Alternatively, a flame sensor might be impacted by an accumulation of combustion by-products such as carbon.
Burner jet blockage
These jets are used to light up the gas as part of the start-up process when you want to use the heating.
Since the burner jets (the component that generates the jets) have tiny openings, small pieces of fuel-burning debris can result in a blockage, thus preventing your boiler flame from lighting up and causing an F4 error.
Malfunctioning ignition lead
Another potential explanation is that the ignition lead (used to help light the gas as it enters your boiler) is not functioning correctly. This would in turn cause an F4 fault code.
Your PBC is faulty
If all else fails, a heating engineer should check your PCB. The fault may be with the PCB itself and of all the error codes it could trigger as a result, it happens to be F4.
Can I Fix an F4 Fault DIY?
Unfortunately, you cannot fix an F4 error by yourself. Instead, you’d have to hire a Gas Safe Register engineer.
This is because of the nature of the repairs/replacement work required.
Moreover, it would also be against the law as only certain boiler repairs are suitable for a DIYer (e.g., topping up your boiler’s water pressure).
Cost of Hiring a Heating Engineer to Fix an F4
The average price of hiring a heating engineer in the UK ranges from £30 per hour to £60 per hour.
However, this would depend on a range of cost-affecting factors. For instance, labour prices differ from region to region.
You’d find generally higher costs in the southeast of England but below-average labour prices in places such as northern England and Scotland.
Here are the estimated costs of specific repairs/replacement jobs for an F4 fault:
Fixing or replacing a faulty gas valve
£230 to £420Fixing wiring or connection problems
£60 to £120Repairing or replacing a faulty/impaired flame sensor
£230 to £430Fixing a faulty PCB
£60 to £240Replacing a faulty PCB
£490 to £800Replacing an entire boiler - £800 to £2,800*
*The reason we’ve featured ‘replacing an entire boiler’ is that if your boiler is beyond economic repair (BER), it would be more financially sensible to have it replaced.
For example, let’s say you need to have a faulty PCB replaced or even a faulty/impaired flame sensor.
In either of these cases, you’d be spending hundreds of pounds. So, if your boiler is particularly old or worn down, it would probably be best to just have it replaced.
However, with a boiler replacement being expensive and considering the ongoing cost of living crisis, many households would consider it out of the question.
However, the good news is that it’s possible to purchase a boiler on finance. In other words, households can buy now and pay later.
Usually, this involves paying interest, but zero-interest options do exist.
Related reading:
Will my Warranty Cover Repairs or Replacement Work for an F4 Fault?
Like with most boiler warranties, Viessmann only covers repairs or replacement work if the issue had been the result of an issue with the material or the manufacturing process.
Beyond that, even if it can be shown that a manufacturing or material defect is at play, you need to double-check that your boiler warranty is still active.
After all, Viessmann’s boiler warranties will vary from boiler to boiler. And, what’s more, certain Ts and Cs must be followed to keep the warranty active over this timeframe.
For example, to ensure your Viessmann boiler stays active, it has to be serviced annually by a Gas Safe Register engineer.
If you’ve any doubts regarding the status of your Viessmann warranty, you should contact Viessmann directly.
You may be interested in Gas Engineer Allen Hart explaining the most common boiler problems and how you can resolve them in the video below:
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