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The expander has never used PCF8574 because it only has 8 channels and expanders have always had 16 channels. The first version of OpenSprinkler used two PCF8574s on the main controller, one on the top board and one on the bottom/driver board. The bottom/driver board is not expander.
The code that handles expander is here:
because it uses dynamic typing (i.e. expander[i] itself carries the type of the I2C chip), you will have to modify code in gpio.cpp:
the simplest hack is to remove the second Wire.write:
see if it works for your PCF8574.