Update table

Hi:

I need to update table with following condition:

Update t1
Set t1.col4 = 'U'
from t1 join t2 on t1.id = t2.id
Where t1.col1 <> t2.col2
t1.col2 <> (select t3.col1 from t3 join t2 on t3.id1 = t2.id1 join t3 as p3 on t3.col2 = p3.id1)

Here t3 table is circular table...

This code is not updating the 1 table where the col2 has changed...

Any help would be greatly appreciated... If more info is needed please let me know....

Thank you very much,
sripri96