When the F4 fault appears on your Viessmann boiler’s display screen, it has locked out to prevent running unsafely.
This prevents internal damage and potential gas leaks, as the F4 fault can indicate issues relating to gas supply. You will therefore need to call a registered Gas Safe engineer to investigate the problem.
This article will help you to understand what may have caused the issue, and what repairs and costs you can expect before it will be fixed.
What Does the Viessmann F4 Fault Mean?
No flame signal is present. This refers to the flame which is needed to burn the fuel which flows into the boiler unit, which is then used to heat water.
As your boiler runs through each stage of the heating process, it sends a signal to the PCB (Printed Circuit Board).
In the case of the F4 fault, the PCB has not received a signal that a flame has successfully been lit. In order to prevent the boiler from continuing to run, potentially risking internal damage and gas leaks, it has locked out as a safety measure.
What Can Cause the Viessmann F4 Fault?
Dirty or Faulty Flame Sensor
This is the part responsible for signalling to the PCB that a flame has been lit. As a sensitive electrical component, its function can be impaired by a buildup of carbon or other combustion byproducts on its surface.
The part can also stop working entirely, with a common cause being a crack in the porcelain insulation surrounding it.
If the flame sensor is causing the F4 fault, it is often the case that the ignition process is actually taking place successfully, but not being recognised. You may have noticed that the boiler has started but shuts off very quickly if this is the case.
Issues with Wiring or Connections Inside Boiler
Every electrical component within the unit is wired up to the PCB, in order for it to accurately receive signals that each stage of the heating process has taken place.
These connections can become loose due to vibrations within the boiler, which can just be a product of wear and tear over time.
However, if you have recently noticed loud vibration sounds, it’s possible that a larger component such as the pump is faulty.
Electrical components can also cease to work entirely, either due to general malfunction or external damage. Within boilers, this is most commonly caused by moisture from internal leaks.
The smallest drop of water can damage some electrical components as they are so delicate.
Stuck Gas Valves
These valves control the supply of gas into the boiler. They remain closed until the boiler begins the heating process, then slide open to permit gas to flow.
If they become stuck in the close position, the boiler will attempt to ignite but no flame will be lit as there is no fuel available.
The sensor will therefore be unable to detect a flame and signal this to the PCB, which will display a fault code such as F4.
Malfunctioning valves can become stuck in a fully or partially closed position, either intermittently or totally. This is more common in older boilers, as the valve is a mechanical part which can eventually break down.
Blocked Burner Jet
The jets are the components through which gas enters into the boiler ready to be burnt, once the gas valve has successfully opened.
As their openings are so small, they can become blocked with even tiny amounts of carbon and debris from the fuel-burning process.
This prevents the necessary amount of gas from flowing freely into the boiler, which will eventually affect its capacity to produce a flame.
Faulty Ignition Lead
These parts are responsible for lighting the gas once it is flowing into the boiler. If they are unable to do so reliably, there will be no flame for the sensor to detect.
As electrical components, they can lose power of their own volition, or face issues related to faulty wiring and connections as detailed above.
Damage to these components is also a possibility, most commonly caused by water within the unit.
As the PCB is responsible for reading all electrical signals, any fault with it can interfere with its capacity to confirm specific processes such as the flame being sensed.
Issues that arise with the PCB are common to the boiler’s other electrical components, namely loose connections, electrical failure, and water damage.
You may have noticed other issues with the boiler’s running if the PCB is faulty, such as cycling on and off very quickly.
Can I Fix My Own Viessmann Boiler With the F4 Fault?
There isn’t a DIY fix for the F4 fault. As it requires an inspection of the internal components of the boiler, it must legally be undertaken by a registered Gas Safe engineer.
Due to the nature of the fault, there’s also the risk of gas leaking into the unit and home, so the safest thing to do is call an engineer straight away.
How to Fix a Viessmann F4 Fault
If The Flame Sensor is Dirty or Faulty
If there is corrosion or a build up of carbon on the sensor, an engineer will be able to safely remove the part and clean it, using a material such as steel wool.
Provided there is no significant damage to the sensor, the fault will be cleared once it has been cleaned and replaced.
If the porcelain insulation has cracked, or the part has otherwise ceased to work, it will need to be replaced. The cost for this can vary depending on the model of your boiler, but usually you can expect to pay less than £75 for the part itself.
If There is an Issue with Wiring or Connections Inside the Boiler
It may be possible for an engineer to notice loose connections with a visual inspection, but if the fault isn’t immediately clear they will use a tool called a multimeter.
This takes resistance readings from electrical parts which can then be cross-referenced with the appropriate measurements for each component.
If specific parts have come loose or ceased to function, it may be possible to reconnect or replace them without significant cost.
While the price of parts is minimal, labour costs will depend on the time taken to diagnose a fault and how fiddly the repair is.
In the case that water damage has occurred, your engineer will also want to find the source of the leak to prevent further issues.
The heat exchanger and pump are common culprits for springing leaks, and can be costly to replace, sometimes also requiring services such as a flush to remove limescale or sludge which have caused the damage.
If Gas Valves are Stuck
Valves can sometimes be freed from their stuck position without replacement, but they’re liable to have the same problem again, particularly in older boilers.
An engineer is likely to suggest replacing the part instead to prevent repeat issues. The cost of gas valves varies depending on the boiler model, but a rough estimate for replacement is £200-250.
If the Burner Jet is Blocked
This is a simple issue to diagnose and fix, and is unlikely to require a replacement jet or burner. Your engineer will be able to remove the affected jet, clean the carbon and debris causing a blockage, and replace it.
As long as no other issue is preventing the free flow of gas, the problem will now be fixed and the fault code will clear after the boiler has been reset.
If the Ignition Lead is Faulty
Issues with this part can sometimes be obvious, as an engineer will be able to see that the gas is flowing freely but attempts to light it either fail or do not begin.
A multimeter can be used to test the power within the part itself, as well as any issues with the wiring that connects it to the PCB. If the lead needs to be replaced, the part usually costs under £50.
If your engineer can see, or suspects, damage to the lead, they will need to inspect the rest of the boiler for issues such as leaks.
Leaking parts can be costly to replace, as well as any other electrical components that have suffered water damage from them.
If the PCB is Faulty
If all other potential causes have been ruled out, your engineer will want to assess the functioning of the PCB. This can also be determined with a multimeter if issues aren’t immediately visible.
Loose connections and wiring can often be repaired, but a damaged PCB will usually need to be replaced entirely as the whole boiler relies on its consistent functioning. These parts are the most expensive at around £500.
If the engineer determines that a leak has caused the damage, they will need to conduct further tests to find the source. If more than one large component needs to be replaced, it may be more cost effective to have a new boiler installed.
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