Upon arrival I found front unit locked out on high Limit. Reset and tested. Found very poor airflow, almost no return air and a lot of discharge pressure. Traced return duct in lobby and found dampers open. Traced supply and found main Damper 80% closed. No idea if it was left that way or vibration caused it to close. Opened all of the way and secured lock so it stays open and I could hear air pressure greatly reduce. Tested heat and ac operations afterwards and found no additional problems. Unit operating normal.