FLO message started the adventure; surfed enough to believe it was a pressure switch. It was replaced. FLO error still. A friend suggested that the 2 weak jets might be the problem. Since they often 'blow off the tub' maybe I had reinstalled the wrong. Nope.
I tried 'jumping' the pressure swtich and it worked for about 2 min until I got a HILI error. reset breaker & tub. Got a HI message (heater is not working) reset again. Checked heater for water inside. Got soaked, OK.
Observations:
after reset no FLO mesg for 1 sec, until filter symbol comes on.
Circulation pump is 'warm' as if it's working; doesn't make any sound or vibration though.
Air in top hose of flo-thru heater. If I pull off the hose the air is pushed out by water; reinsert hose on heater- OK until pumps run and there is more air in top hose.
No water runs through the hoses that I can see.
I forced water down the hole that goes to the circualtion pump and for 1 sec FLO diasppeared and a big air bubble rose from the bottom of the tub. Checked the heater hose and again full of air.
See below for photo.
Circ pump is 'A O Smith' Tub is 2 1/2 yrs old, inherited with house, only 6 mos old @ posession date.
Thanks!

Where is the air you reference? The heater assembly is in the right side of your picture, are you saying there is air in the top tube of the heater at the 90?

Yup, the top hose of the heater get an air bubble when I force water through with the plunger into the circulation pump.
An air bubble also comes out at the bottom of the tub where ozonated water usually comes out.
Water level in the 'T' connection of flexible hose leading back to the ABS T connection is stagnant, only to be re-filled when I disconnect the top heater hose to release the air bubble- It fills all lines with water until I plunge again
I have bled the water supply lines repeatedly, and there aren't any shut-off valves nor air vents in the wet-end of the circ pump to bleed that line. If I disconnect the ozonator line air seems to 'whoosh' out of it constantly. Normal?
A friend suggested there might be a 'flap' on the inlet hose that might be broken and spring shut (as he replaced on his Beachcomber tub).
Do I have an air-lock in the circulation system? I have tried forcing water from the hose down it and it seems to 'force' it's way back into the tub.
Thought lots of detail might help...
Thanks Martha

Sounds like the little circulation pump in the bottom left of your picture is not pumping. Somehow you need to find out. There should not be any "air" in the tube coming out of the heater and the water should be moving when that little pump is on.

The pump is 'working' though- it gets warm/hot when I flip the breaker on during repair sessions.