Why wont my car go over 30 mph? on 2003 Nissan Altima

When putting pedal to medal my car wont go over 30. I replaced the transmission and nothing. I also replaced the speed sensor and nothing it is doing the same thing. Diagnosis said it was the tranny and it still does the same thing someone help no one can seem to figure out whats wrong :(

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2 answers
check for plugged cat converter
agree with goodguy