02 sensor won't thread in
 

Removed old o2 to replace with denso. old one came out difficult. Went to thread in the new one and the threads just won't catch. anyone else have this problem?

Some do some don't, you need to chase/tap the threads in the pipe bung.

The O2 bung may have damaged threads. The threads would have been damaged due to galvanic corrosion "welding" the threads together.

You may have to have a new bung welded to replace the old one. Be sure to install the new O2 sensor with Anti-Seize on the threads. Use care in not getting Anti-Seize on the O2 sensor element.

would getting a new cat be easier

Easier? No. Not really.

ok so I remove the cat, then tap it. what does that mean?

Don't even remove the cat. Just get a tap the right size, run it through the hole a couple times to clean up the threads, and you are all set.

never used a tap before but the hole is toward the top, is it still possible without removing the cat?

Yes, you don't need to take the cat out.

18mm is the size,

Tool link http://www.northerntool.com/shop/too...6166_200396166

that tool looks very self-explanatory. thanks