Viessmann F6 fault

February 15, 2023

When the temperature drops below or rises above a certain point, the F6 fault code is triggered in a Viessmann boiler. In a conventional boiler, the F6 fault would be caused by an over-temperature condition. In the Viessmann System Boiler, the F6 fault is triggered when temperatures are not within a tolerance of each other, and the boiler cannot maintain temperature throughout its heat exchangers. Read more about the F6 fault!


What does the F6 fault code mean on my Viessmann boiler?

The F6 fault code is a temperature fault, meaning that the boiler's temperature is too high or too low.

In this case, it is most likely that the boiler's thermostat has failed. However, there are other possible causes as well. For example, if you have recently installed a new radiator in your home and it is connected to your boiler, this may cause the F6 fault code to appear on your control panel.

If you suspect your thermostat has failed, you should contact a professional heating engineer to diagnose the issue and replace the faulty part.


Common Causes of Viessmann F6 fault

Several issues can cause a Viessmann F6 fault. The most common cause include:


A damaged thermistor or temperature sensor

This is the most common cause of an F6 error. The thermistor is a sensor that measures the temperature inside the compressor. If it is damaged, the compressor will not be able to function properly and will generate an F6 error code. The most common way this can happen is if there is dust or dirt on the thermistor, which can prevent it from working properly. You can clean the thermistor with a brush or compressed air. If this doesn't solve your problem, you will need to replace it with a new one.

The compressor has a short circuit failure

A short circuit occurs when electricity travels along an unintended route through a component in your appliance, such as your compressor. The result is that power flows through wires that were not intended by the manufacturer and may cause damage to other parts in your appliance or even start a fire! To fix this issue, you will need to replace the parts affected by short circuits to restore normal operation.

The pressure switch has failed

This switch monitors the pressure in the circuit and turns the boiler off if it drops too low. If this fails, the boiler will continue to operate even though it may not be getting enough oxygen to burn properly.

Incorrectly set pressure switch

If you have adjusted the pressure switch incorrectly, it will shut down the boiler prematurely. This can cause the boiler to run inefficiently and lead to carbon monoxide poisoning if there is no draft through the flue gases to ventilate them adequately.

The boiler is not receiving enough oxygen for combustion

A bad burner orifice plate could cause this condition. The orifice plate regulates the amount of gas coming into the furnace and controls its flame intensity. If it's partly clogged, it will restrict the amount of gas going in, which will cause an increase in gas pressure and lower temperature readings. This can also cause carbon monoxide buildup because there isn't enough fuel to combust fully, which leads to incomplete combustion and carbon monoxide production. The solution here is replacing the orifice plate with a new one that will allow more gas flow and help lower your temperature readings.

The thermostat has failed or is set too high or too low

Check the temperature setting on your thermostat and ensure it matches what you want to heat your room too. If it's set too high, you'll get an F6 fault when the boiler switches off as it's trying to cool down too quickly, which can lead to heat damage if left for long periods.


A faulty control board (controller) or circuit board

If you have checked your thermostat and it seems to be working correctly, then there may still be a problem with the control board (controller) or circuit board. If any other appliances in your house are causing the heating unit to trip out, such as an air conditioner, then this may also be causing issues with the heating unit itself.


A misaligned or damaged PCB track

The F6 fault is a misaligned or damaged PCB track. The track is connected to the heating element and allows the current to flow from the element to the thermostat. If this track is damaged, it will prevent the current from reaching its destination.


An incorrect or faulty firmware update

Another common cause of this fault is an incorrect or faulty firmware update. When you install new software onto your Viessmann boiler, there may be issues with the code used in this installation, and this can cause numerous problems within your boiler, including F6 faults.


Is it likely I’ll need a new boiler if it’s displaying the F6 fault code on the display panel?

The F6 fault code is related to the boiler’s temperature control system. If it’s displayed, it means that the boiler has sensed a fault with one of its sensors.

In this case, you likely need to replace the sensor. There are two types of temperature sensors on your boiler – the first is called a thermistor, and the second is called a PT100. Both sensor types are fitted between the heat exchanger and the main circulator pump.

Your boiler will have a separate electrical connection for each of these sensors. So to check which one needs replacing, simply disconnect them from their terminals and then reconnect them again in turn. You should be able to find instructions on how to do this in your owner’s manual if you can’t remember where they are located on your boiler or if you don’t have an owner’s manual.

As soon as one of these connections breaks contact, i.e., when you disconnect it, your boiler should display an F6 error code on its display panel – which tells you which type of sensor needs replacing, thermistor or PT100.



A faulty gas valve can be quite dangerous. The main reason for this is that the gas valve is designed to shut off the gas supply in the event of a power cut or other failure. If it fails, you could be left without heating during cold weather and with no way of getting warm again until the problem is resolved.

The danger also comes from the fact that a faulty gas valve can cause carbon monoxide poisoning. Carbon monoxide poisoning can be fatal, so it's essential to check your heating system regularly and ensure all parts are working correctly. If the pilot light doesn't come on when you turn on your furnace or boiler, then there may be a problem with your thermostat or wiring system.

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