Spa-Quip inline optical water sensor.
If you pool is flashing error "H20", and its full of water and you have no flow issues...then its most likely this sensor.
This sensor is suited for heaters on the SP400, 600, 601, 750, 800, 1000 (does not suit older model heaters) and 1200.
Does not suit the SP54500 or 500A.
O-Ring replacement recommended, part HERE
Please note: These sensors are to be installed finger tight only, do not use tools to install.