My experience with Costco.
1. Purchased inexpensive Hisense TV.
2. Total, warranty. 4yrs. (as other have mentioned).
3. One month before warranty was to expire, TV, had "dark" areas on screen
that would move around.
4. Did everything, OP, did. ie. Called Costco, then Hisense mfg.
5. Did the "taking" pictures of serial number, pictures of the screen.
6. Many phone calls, emails, and more pictures. As requested by Hisense
customer service.
7. End result. After about 6 weeks. Received brand new TV from mfg.
Advice: I've noticed this is the "procedure" one has to follow to get a TV
replaced/repaired. You have to have patience. And expect it will
take time and a lot of correspondence.
Costco, IMHO. When it comes to electronics, has the best customer service.
ie. 90 day return.
8. Weird note. I moved the "broken" TV to another room. Turned it on.
and the "dark" cloud was gone. TV working fine for years
now.
I know a lot of the "solder" joints are around the "edge" of the TV.
So, grabbing the sides/edge of the TV, when moving, probably
reconnected some loose connectors/solder/wire joints...