Thanks for the information caplanda.
First I note the preliminary version of the data sheet (0.8 D1 Jan 2018) indicates on page 4 that both Modbus and PWM are not yet supported. So it looks as if one is left with just I2C SCL / SDA mode of information transfer for the time being.
So I coupled the sensor up to the SCL/SDA pins on my Uno R3 board but got no response. (By the way the reference to pins A4 & A5 are to Uno R1 boards which don't have the pin outs beyond AREF.)
I then had a brain wave and set up a separate power supply to the sensor. The data sheet claims it's peak draw is 75mA which I though a Uno could just about manage through one pin. That gave me an encouraging but transient orange glow from the sensor's light grey area when I switched the power on. But still no response.
I then spent several hours tracking through all sorts of things and eventually turned up in SparkFun_SCD30_Arduino_Library.h (well SparkFun_SCD30_Arduino_Library.cpp) a reference to the fact that this coding was created using Ardunio 1.8.5; my version was 1.8.2 of course. Still no response after I updated and recovered all my added libraries. (I am most grateful to whoever put the warning in the uninstall coding to check one's libraries.)
Then on rereading your posting I noted the comment about leaving the SEL pin floating. So I went back to the data sheet which says "Pull to VDD for selecting Modbus". So I thought what about pegging it at ground so it won't be confused. And lo and behold, it worked. I get an orange glow every couple of seconds and a read out of "co2(ppm):556 temp(C):28.2 humidity(%):50.1" on the Serial Monitor. It even reacts to breathing on it in both CO2 and humidity.
