(2002 Toyota Previa/Estima/Tarago ACR30, engine 2AZ-FE)
One of my two pre-cat O2 sensors is faulty. The garage I put the car in did not manage to replace it - they told me it is stubborn and that even after applying heat (acetylene burner) it did not come off. They were able to turn it only a bit and than it stopped.
They did not want to apply brute force as the thread in the exhaust manifold could be stripped.
I believe they were kind of in a hurry (I asked them to replace the sensor after finishing a long list of jobs and they already had other cars ordered in), so they may have not applied any kind of penetrating oil.
My question is - am I in a danger of stripping the manifold thread if I apply extensive force on the sensor (provided that I won't strip the sensor's hex head)?
EDIT 2017-10-08: After a day of spraying the O2 sensor with WD-40 Specialist Penetrant, I set myself to remove the sensor. Took me about an hour of slow work with 22 box wrench, vice grip and a hammer (I didn't remove the heatshield). The thread of the old sensor was indeed stripped (well, to be precise, its outside half), so the material must have remained in the exhaust manifold's thread.
Then came the worse part of the job - reviving the thread with a tap. I went really slow and after basically three passes I was able to rotate the tap by hand. Tried inserting the new O2 sensor but when I started to feel the resistance, I removed it and decided to first screw another old O2 sensor into the manifold - for case I damage its thread. I really wasn't sure but had basically nothing to lose so I applied more force and finished screwing the sensor. Ran the engine for a while to test it does not leak. Everything was surprisingly OK so I removed the old sensor and now only had to put the new one in. After couple of minutes, voilà, it was there :)