1999 Toyota Tacoma P0171 Reading

99 tacoma, took it to advance to get the check engine light read. P0171 reading. Replaced the O2 sensor before the cat. I haven't had the check engine light cleared to see if the O2 sensor worked. However after I replaced it, starting it seemed a little rough, while previously it was not really having any issues. Could not having the check engine light cleared cause this? What could other issues be?