Viessmann F4 Fault Code – Likely Causes, And Possible Fixes

December 2, 2022

What Does the Viessmann F4 Fault Mean?

Viessmann boiler F4 fault is one of the most common issues that plague Viessmann boilers, and it's such a tricky problem that many people have trouble identifying it in the first place. The issue is often caused by problems with flame recognition, which means that the sensors don't recognize when sufficient flame is present.

If your Viessmann boiler displays the F4 fault code it is vital for you to fix it with a professional Gas Safe registered repair service as soon as possible before more severe damage occurs.


What Can Cause the Viessmann F4 Fault?

The F4 error code can cause several different problems, but they all share the same symptoms. The boiler will not heat up, and no visible flame is created. Many things can cause this.


Dirty or Faulty Flame Sensor

The flame sensor is a small device located near the boiler burner. It is responsible for detecting the presence of flame to control the ignition cycle of your boiler.

The sensor also detects when there is no flame, which will cause the boiler to shut down. The sensor can become dirty over time, and this may result in it failing to detect a flame, causing the boiler to lock-out.

Or, it might fail to detect a no-flame condition. This will cause your boiler to fill with gas if the gas valve hasn’t been told to close.

Issues with Wiring or Connections Inside Boiler

The wiring and connections in the boiler are often the first places to look when troubleshooting a Viessmann F4 Fault. The most common issue is a loose connection between the gas valve and the electronics. If this happens, we may be able to fix it by simply tightening the wires together.

Stuck Gas Valves

The gas valve regulates how much gas passes through to the burner, which in turn controls how much heat your boiler produces at any given time

Sometimes these valves can get stuck in one position (either fully open or fully closed) and cause problems with your boilers operation — including causing F4 faults like this one!

Blocked Burner Jet

The burner jet is blocked with soot or mineral deposits. This could be caused by an incorrectly adjusted flame or poor gas quality. When you turn on your boiler, the burners should ignite and produce a bright blue flame approximately 4 inches (10cm) in height. If the pilot flame is too weak, the sensor will not recognise a flame, and your boiler won't work correctly.

If the pilot flame is too strong, it will cause carbon deposits to build up on the burners and block them from working correctly.

Faulty Ignition Lead

The ignition lead is a wire that runs to the spark probe and is responsible for igniting the gas at the burner. If this wire is broken or shorted, it will cause an F4 error code. This can be easily tested by removing the connector from the ignitor and testing with a multimeter. If there are no continuity readings, replace the ignition lead.

Faulty PCB

The PCB (printed circuit board) acts as the brain of your boiler and is responsible for sending signals to different components to control them properly. This can be tested by disconnecting each component from its respective terminal on the PCB and testing with a multimeter, which should give you continuity readings if all of your components are working correctly.


Can Viessmann F4 Fault be fixed on a DIY basis?

The F4 fault on your Viessmann boiler is a serious problem. It means that the flame sensor has failed, and the boiler cannot detect whether there is a flame present in the combustion chamber. Several issues can cause this, for example, if the flame sensor has warped or become damaged or if it is blocked by soot deposits or debris.

If you have an F4 error code displayed on your Viessmann boiler, you will need to call in a Gas Safe registered engineer. They will be able to assess the issue and carry out any necessary boiler repairs.

If you try to fix this issue yourself, there is a high probability that you will damage your boiler's electronics beyond repair; this will result in having to purchase a new boiler installation, which could be expensive!

How much will it cost to replace the PCB on my Viessmann boiler?

When the boiler's PCB reaches the end of its lifespan, a fault code will appear on the control panel of your Viessmann boiler. It will display an error code such as "insufficient flame recognition." This indicates that the boiler is not firing when it should be and that there is a problem with the PCB or wiring. The current cost for replacing the PCB in a Viessmann boiler is around £400-500, including parts and labor. However, the price can fluctuate depending on your location, the stove model, and what options are included.

Is it safe to reset a boiler?

Resetting a boiler is a simple process that anyone can do. The process involves opening the valve and allowing water to drain from the system. Once the water has drained out of the system, you close the valve and turn on your heating system as usual.

You should always consult a professional if you have any doubts about resetting your boiler, but for most boilers, there isn't anything to worry about. Is it safe to reset a boiler? The short answer is yes, but you need to get some advice before doing it yourself.


F4 fault is a serious problem. It may require some maintenance to get fixed, and if you don't do it in time, your boiler will either stop working for good or stop working until you take action. A warning like this might alarm some homeowners into rushing their repair job, but it's more important to take your time when working with any gas appliance. Otherwise, you could cause even more damage and end up in a sticky situation. An annual boiler service should help to spot the Viessmann F4 fault before it develops.

At Heatsy, we’re trained and accredited by Viessmann and have years of experience working with their boilers.

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