AO2
Morning all,
The AO2 remark is just stating that the METAR was produced by an automatic surface observation station (ASOS) that has a precipitation sensor/discriminator, plus in this case it also has a lighting detection system which produces the notes in the RMK section.
The AUTO designator in the METAR means that the reports are generated and distributed without any human oversight. When there is no AUTO it just means the met man/woman is in the building to check the auto-generated reports before they are issued, and can add any RMKs he/she sees fit.
If the system requires maintenance the whole report will be appended with a dollar "$" sign as B777FD says.
Buzzz