Gas cap and purge cylonoid has been replaced throttle body replaced....10 days later....tighten gas cap back on

What is your question?

Check engine light -- replaced purge cylinoid....tighten gas cap light on within week.....brand new cap for cts 08.....3 wks later had replace throttle body....Cadillac dealer now wants charge us for another diagnostic 10 days later....was a problem the first time diagnostic was done....why didn't it show....at our wits ends.....why didn't it show up week ago the problem going on a month..........any answers or sugg

What makes this problem better or worse?

Nothing

How long has your 2008 Cadillac CTS had this problem?

Month or lil more