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You installed pyOBD: OBD-II compliant car diagnostic tool.

pyOBD expects the device /dev/ttyU0 that should be created when you
connect the OBD-II cable.

You need these lines in /etc/devfs.rules to make the device readable
and writable by you. If your user is in the group users:

[localrules=5]
add path 'ttyU*' mode 0660 group users

You also need this line in /etc/rc.conf to allow localrules to take effect:
devfs_system_ruleset="localrules"
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