Just a footnote... Just this morning I hit a case of i2c scan failing to see a device on the i2c bus, that i2cdetect -y 1 found just fine. I even tested the sensor via python3 just fine as well. It was a HTUD21 sensor at address 0x40. The i2c scan node reported and empty array of devices on output 1, and absolutely nothing came out of output 2 when the issue occurred I swapped in a BME280, and i2c scan node worked fine. So it might be an issue specific to only some sensors? I tested with BMP280, BME, MCP9808, etc., no issues. But i2c scan still can't seem to see an HTU21D or SI7021 sensors, at least on my setup that is. They use the same command set, so the same python3 script can read either, interesting that the i2c scan node fails to see either of them so something about the address being 0x40 or that they have the same command set maybe part of the issue?