changing a catalytic converter 

Recommend watching at 1.5x
My old workhorse needs a new cat - 2008 Honda Civic Sedan
Throwing P0420, P420 code
Got the new cat off of ebay for $100.
DISCLAIMER: This is a cheap ebay cat! These cats do not last nearly as long as an OEM part or even a comparable autozone part. As seldom as I drive this car, I expect about 2 years out of this part, which I am perfectly happy with. If you drive your vehicle daily or close to it, you may consider getting a higher quality cat, especially if your cat is difficult to change.
Pre-soak the bolts with PB Blaster, or liquid wrench, or equivalent. WD40 is NOT made for this. WD40 can help in light applications, but a real penetrant will work much better. Exhaust bolts like to break - don't chance it.