I picked up a TRD header. Getting around to installing. On the downpipe there is a O2 sensor bung. My corolla doesn't have a sensor located down there. What's the use of it on the downpipe. The reason I ask is there's a broken bolt in one of the holes and I'm not sure if I should just weld it shut or fix it.

extend the O2 sensor wire to so you can mount it there. they want to get the reading from all 4 cylinders

Bung after the final merge is usually for an aftermarket wide band O2 sensor.
The narrow band sensor (stock sensor) uses a bung on one of the primary pipes
about 12-18 inches from the head. If you don't have a WBO2 sensor, block off
the downstream bung. But if you don't have a narrow band bung in the engine
bay, you could do as morgan suggests.

Cheers... jondee86

ah ok so that's for the stock narrow band o2 sensor. I went carb. I'm thinking i'll fix the broken stud, put a block off plate and in weld another bung on that pipe for a wideband for tuning purposes in the future.

you could always weld the bung onto the block off plate.. just sayin.