Because this thread is one of the top results for "jacuzzi flo2", I wanted to include my problem and corrective action.
First: on a Jacuzzi, the FLO2 error simply means the electronics are informed of "flo" when the electronics don't think there should be flo. The error can be caused by a stuck sensor as most of this thread discusses, but can also be caused by:
1) A “short” in the sensor circuit (anywhere from the circuit board all the way to the sensor)
a. So the main board may need to be replace or the sensor may need to be replaced
b. The sensor assembly for my Jacuzzi also contains the wires.
2) The circulation pump running when the electronics think it shouldn’t
a. This is what caused my issue. When the previous pump fried, it took out the main fuse and threw the breaker. It also melted the relay contacts together causing the new pump to run constantly.
b. As described, there was flow to the sensor at all times and the electronics reported this as a flo2.
I hope this helps someone out.