mount vernon AE vacuum switch error

[buck 01]

Stove is almost 2 years old and at the end of last season the vacuum switch error kept coming on yes very clean I have it vented out the back of stove with a T and to a 90 elbow through the wall 2ft to another T and up 8ft through the roof and it about 2ft above roof with a rain cap had a dealer look at it and tested every thing and said it was fine but still giving error any help would be nice. only does it at start up and after it autocleans

Thanks,