I got it working. The sensor has 2 modes:
Q&A
Active
In active mode it should send a result every second but I only get FFFF...
I only got it to work in Q&A. I have to send it the 9 bytes command and it responds with the readings
I didn't use the library, just the plain SoftwareSerial directly in the info
Also I had to connect sensor TX to Arduino RX and RX to tx, but maybe that's the normal way
thank you for your answers