hello I will not answer about the PCF8574 (who is sometimes very strange !)
but to the question "how to see if a I2C unit is alive"
I use for a long time, a very simple programm TEST I2C
see here : http://mbouget.perso.neuf.fr/telecharge.html
you can search, read,write on a I2C line (from a window/dos command line)
I use it all the time to check my "I2C sentence"
hope this will help other PIC user from this super Forum list !
francois F1CHF