I don't think that there is any documented way to check that in the (coding) menu. If you use third-party software (unsupported by Viessmann) that logs the behaviour of the burner control over the OptoLink interface you could easily see if the modulation does no longer go down to 11% of power. There is quite some information about the OptoLink interface and software to use it for data logging in German on https://github.com/openv/openv/wiki
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