What is this thing and do I need it?

[Fosty]

Because mine is missing

[04pathse]

I think its one of the A/C service ports, Iow side I believe, the cap is to protect the service port from getting dust, dirt or water in it. You should be able to find a replacement cap at any auto parts store, junkyard, or
they have the set low, and high on ebay for around $10 shipped
https://www.ebay.com/itm/181890114983?f ... SwM4xXcQKT

[Fosty]

I think its one of the A/C service ports, Iow side I believe, the cap is to protect the service port from getting dust, dirt or water in it. You should be able to find a replacement cap at any auto parts store, junkyard, or
they have the set low, and high on ebay for around $10 shipped
https://www.ebay.com/itm/181890114983?f ... SwM4xXcQKT

Thank you!