It took me only 15 minutes to take photos and upload them to ChatGPT, have it recognized and then get catridge ordered from Amazon and thought I could avoid $150-$200 of plumber call (saving of about $100-$150). But this nut is stuck. Plier wrench could not open it.
Do I need to spray with WD40? or venegar?
Some youtube videos suggest to cut it (https://www.youtube.com/shorts/5zvPuryfYZE ), but this is about $100 part on Amazon. There is risk to cut into the thread underneath
https://www.amazon.com/Delta-Replacement-1600-Bonnet-RP6079/dp/B0CZ3N31RJ?
HD
$15 but it would take a few days to get it.
https://www.youtube.com/shorts/j90pI-TU6mA
this is a trick I could try.
https://www.youtube.com/shorts/8dJOQ1x24IE
One more trick. This trick worked for me. I do not have rubber gasket so I put alumnium tape (hot exhaust tape) and it can easily pushed into groove on top of bonnet nut.
This puller helped me pull this out. Two point contact tool works much better than single point contact tool like plier to pull this catridge out.