Increased consumption

Hello.
This is not to be a debate on consumption.
My 3.6 has been consuming a lot of gasoline for a few days, according to the BC display as a real ernorm.
At 120 km/h 15-18 l in a low load range.
Strange is that the problem is not constant. At a distance of 600 km the problem occurred several times and disappeared again.
The engine temperature is in the normal range.
The lamda probe is almost 50tkm in it and does not behave noticeably – no pulsating acceleration
Mistake reading unfortunately did not provide any information.
Before I change lamda now, my question, it can be a simple mass problem of lamda
be or one of the two temp sensors. Which exactly is responsible for the control?
Or a completely different mistake?

Greetings Davut