What does the Viessmann F4 fault code mean?
F4 on a Viessmann boiler means there is no flame signal. This is the flame required to burn the fuel that flows into the boiler unit. The boiler then heats the water which is passed around your central heating system and to your hot water taps.
Your Viessmann boiler sends a signal to the PCB (printed circuit board) as it goes through each stage of heating.
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The F4 fault means that the PCB did not receive a signal that a flame had been successfully lit. To prevent the boiler’s continued running, possibly exposing it to internal damage or gas leaks, it has been locked out.
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What Causes the Viessmann F4 Fault on boiler?
Faulty or Dirty Flame Sensor
This part is responsible for indicating to the PCB that the flame has been lit. It is a sensitive component that can be affected by carbon buildup or other combustion byproducts.
Sometimes the part may stop functioning completely. This is usually caused by cracks in the porcelain insulation around it.
Viessmann boiler F4 faults are often caused by the flame sensor. This is because the ignition process is happening successfully but is not being recognised. If this is true, you may have noticed that your boiler shuts off quickly after it has started.
If the Flame Sensor is Faulty or Dirty
An engineer can safely remove the sensor if there is corrosion or a buildup of carbon.
If there is not any damage to the flame detection sensor it will be repaired and replaced.
It will be necessary to replace the porcelain insulation if it has cracked or stopped working. This will vary depending on your boiler model, but you should expect to pay less than £80 for the part.
Problems with Wiring and Connections inside Boiler
The Viessmann boiler F4 fault code could mean there is an issue with the electrics in one way or another
Each electrical component in the unit is wired to the PCB to receive precise signals that indicate which stage of the heating process for the hot water occurred.
These connections may become loose from vibrations in the boiler. This can be due to wear and tear.
If you’ve noticed loud vibrations recently, it could be that the pump or another larger component is defective.
Sometimes, electrical components may stop working entirely due to malfunctions or external damage. This is most common in gas boilers and is caused by moisture from inside gas supply leaks.
Because they are delicate, even a tiny drop of water can cause damage to some electronic components.
If there is a problem with the wiring or connections inside the boiler
Engineers may be able to see loose connections by visual inspection. However, if the Viessmann boiler fault codes are not immediately apparent they will use a multimeter.
This allows you to take resistance readings of electrical parts, which can be then cross-referenced with the correct measurements for each part.
It may be possible to replace or reconnect parts that have become loose or no longer work without significant expense.
Although parts are inexpensive, labour costs can vary depending on how difficult the repair is and the time it takes to diagnose the fault.
Your engineer will want to locate the source of any water damage to avoid further problems.
Common causes of leaks are the heat exchanger or pump. These boiler components can be expensive to replace and may also require services like a flush to remove limescale and sludge.
Stuck gas valves
These valves regulate the flow of gas to the boiler or low gas pressure. These valves remain closed until the boiler starts heating, and then they open to allow gas supply to flow.
They can get stuck in a tight position so the boiler may attempt to ignite them but there will not be any fuel.
Therefore, the sensor will not detect a flame and will not signal it to the PCB. The fault code F4 will be displayed.
Malfunctioning valves may become stuck in an intermittently closed or completely open position. This happens more often in older boilers because the valve is a mechanical component that can eventually fail.
Gas Valves that are stuck
Sometimes, valves can be removed from their stuck position by replacing them, but it’s possible for them to experience the same problem again in older boilers.
To avoid repeat problems, an engineer will likely suggest that the part be replaced. Gas valves cost will vary depending on the model of boiler, but an estimate for replacement would be between £210 and £260.
Block Burner Jet
Once the gas valve is opened, the jets allow gas to enter the boiler and be burned.
Because their openings are so small they can become blocked by even tiny amounts carbon and debris during the fuel-burning process.
This stops the gas from flowing freely into boiler, which eventually will affect the boiler’s ability to produce flame.
Block the Burner Jet
This is an easy problem to fix and diagnose. It is unlikely that a new burner or jet will be required. An engineer can remove the blocked jet, clean it of carbon and debris, and then replace it.
So long as there is no other problem preventing gas flow, the problem can be resolved. After the combi boiler has been reset, the fault code will disappear.
Faulty ignition Lead
These parts light the gas when it’s flowing into the boiler. The sensor will not detect a flame if they fail to light the gas reliably.
They are electrical components and can lose power at will.
Water within the unit is often the cause of damage to these components.
Fixing Faulty Ignition Lead
Sometimes, issues with this part are obvious. An engineer can see that the gas flows freely, but attempts to light it fail or don’t start.
Multimeters can be used to check the power of the part and any problems with the wiring connecting it to the PCB. The part will usually cost less than 50 pesos if it needs to be replaced.
If the engineer suspects that there is damage to the lead they will inspect the boiler to determine if it has any issues.
Leakage can lead to expensive repairs, and other electrical components may have suffered water damage.
Faulty PBC
The PCB is responsible to reading all electrical signals. Any fault can cause it to be unable confirm certain processes, such as the detection of the flame.
The PCB can have problems that are similar to other components of the boiler, such as loose connections, electrical failure and water damage.
If the PCB is defective, you may notice other problems with the boiler’s performance such as the heating system cycling on and off quickly.
If your PCB is Broken
After examining all possible causes, the engineer will need to examine the PCB’s functioning. If the problem is not immediately apparent, a multimeter can be used to determine its functioning.
Although loose connections and wiring can be repaired, a damaged PCB may need to be replaced completely as the boiler depends on it functioning properly. These parts cost around £550 and are the most costly.
The engineer will conduct additional tests to determine if a leak is responsible for the damage. It may be cheaper to install a new boiler if more than one component is needed.
How do I fix a Viessmann boiler F4 Fault?
The Viessmann F4 fault code is not a DIY problem. It requires an inspection of internal boiler components and must be done by a Gas Safe registered engineer. If you get a Viessmann boiler sevice every year then this could be caught early.
Because of the fault’s nature, you could cause further damage or gas could leak into your home and unit. It is best to call a Gas Safe registered engineer immediately.
If it can’t be fixed or becomes really expensive it may be more economical to invest in a new Viessmann boiler unless you are still under warranty.
Still got a Viessmann problem? read our other error codes guide such as the Viessmann F2 fault code guide for more information.
