Technical Service Bulletin

Burner is locked out with fault code F6 on Vitocrossal 300 CU3A

Applies to: Vitocrossal 300, CU3A Series (all models)

Important Disclaimer

The information in this bulletin is intended exclusively for licensed heating contractors, and is NOT intended for the end user. This material is NOT intended to take the place of the information contained in the manuals shipped with the product.

Use of this information by unqualified personnel may result in property damage, personal injury or death.

End users are asked to contact a qualified contractor for all installation and service work. If you need assistance finding a contractor in your area, contact your Viessmann Sales Representative, or use our Contractor Locator.

By hitting "Yes" below you confirm that you have read, understood and agree to this warning, that you are licensed and hold the current, appropriate licences to work on said equipment in the location and/or jurisdiction of installation.

No, I don't agree. Help me find a contractor


The burner is locked out, and fault code "F6" is displayed on the boiler control.



Faulty boiler water temperature sensor.



Test and replace the boiler water temperature sensor according to the  Service Instructions  for Vitocrossal 300, CU3A.



The F6 fault code indicates that the temperature values of the dual-element boiler water temperature sensor vary too widely from each other.

This fault code does not appear in the Service Instructions as of this date; updates will be included in the next edition.


Viessmann Technical Services


Bulletin ID:


Date issued:

February 1, 2017

Updated on:

November 24, 2017

Revision #:


How can we help?
How can we help?