My grand prix gt won't start. Any ideas on what it could be? on 2003 Pontiac Grand Prix

I have replaced the battery and cleaned the connections. I also replaced the ignition switch and alternator and the just recently the starter and it still wont start.

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2 answers
does it turn over? if so check for spark and fuel pressure
Will engine turn at all,is it locked up? Try it with drive belt removed may have a componet seized!