Exhaust questions

[xt master]

I have a few questions,I am replacing my exhaust from the manifold back.

1.Are the bolts that hold the down tube to the manifold stamped in place?

They seem to be and I did not want to start hitting them with a hammer to remove them.I would like to replace them if possible.

2.If I can get them out what kind of bolt should I buy?

3.Is there any way to tell if your O2 sensors are good by looking at them?

I dont really have the money to replace them now,but if they really need it it would be easier to do it now.

 

[Bounty__Hunter]

The bolts that attach the downpipe to the manifold are welded to the manifold. Hit them with some PB Blaster for a day or so before removing them from below, I believe they're 9/16".

No way to tell the o2 is bad that I know of, other than a check engine light and a trouble code.

 

[xt master]

The bolts that attach the downpipe to the manifold are welded to the manifold. Hit them with some PB Blaster for a day or so before removing them from below, I believe they're 9/16".

I got the down tube off.2 different size nuts?I wanted to replace the bolts also since they are all rusted,but if there welded in I am not touching them.