New here. CEL question

Hello. I just joined here I have a 2000 GT 5 speed. The check engine light was on when I bought the car, I had an OBD-2 Reader and scanned the CEL. Came back with Banks 1 & 2, Sensor #2. So both downstream o2 sensors. The car has Hedman Headers, and a custom X-pipe with no catalytic converters on it.

Needless to say I have to pass stupid emissions, so I had the catalytic converters put on as well as both the downstream o2 sensors. The "Service Engine Soon" light is still on, and if you know emissions that is an automatic failure.

What I am wondering is, do I need to manually turn the light off, or is it eventually going to turn itself off? I know that unhooking the battery will turn it off, but it might come back on for whatever reason. Just need some help in what direction I should go next. My temp tags run out on the 29th, and I am pretty sure they aren't going to give me another one. I have NEVER owned a mustang before So I don't know the in's and out's. Someone help please, and thank you.

This is extremely frustrating and expensive for a broke college student.