I2c not working on the Pi4 - Home Assistant OS - Home Assistant Community (home-assistant.io)
1) remove the sd card from the pi and plug it into a pc running Linux
2) most likely linux will not mount the partition "hassos-boot" because it is FAT
a) open a terminal window
b) type > blkid
c) in the resulting output, you'll see "LABEL="hassos-boot", note the location, mine is /dev/mmcblk0p1
d) type > sudo mkdir /mnt/msdos
e) type > sudo mount -t vfat /dev/mmcblk0p1 /mnt/msdos
d) now you can open the file /mnt/msdos/config.txt
e) uncomment the line "dtparam=i2c_arm=on"
3) If the hassos-overlay partition mounted, (if not, do step 2 for this partition)
a) open the file: hassos-overlay\etc\modules-load.d\rpi-i2c.conf
b) ensure it has only these 2 lines: (mine already did, likely from when trying "import usb" method)
i2c-bcm2708
i2c-dev
4) replace the sdcard back into the pi and start normally
a) the i2c bus should be functioning now
