No Water Solution... easy, but not for the timid. Read the whole thread before you proceed. Takes about 10 minutes, and you need a 2" piece of +\- 22 gauge wire, a small regular screwdriver, and a regular philips to remove power unit from wall and unscrew the back case.
I have a CPX16. The main power unit is the same for larger sizes, CPX24, CPX36, and CPX48. I have read dozens of comments and threads on the "no water fault".
My unit is out of warranty, but otherwise in excellent condition.
If you have exhausted the usual symptoms, i.e. there is water in the unit, its clean, etc, then the issue may be the sensor in the cel has gone bad, as it is for mine. This is not a serviceable sensor, and the whole cel must be replaced to fix.
There is no "Flow" sensor, only a water sensor, which closes the circuit to ground when salt water is present.
Opening the main power case, breaker power off, find the two wire blocks with the regular set screws. Add a bridge wire from the water sensor (the last green wire in the 7 wire block to the cel, its marked on the circuit board), to the grnd wire terminal in the next block (mine was position 4 from left to right on the 6 wire block, its marked on the circuit board also) leaving the original wire in place. Reassemble and turn power back on. Unit will self test and ... Bingo. Water fault gone. Unit operates beautifully again.
My unit is wired to turn on with the pump timer and will never run without both. If the cel is dry, the contacts won't produce anything anyway as there is no electrical path without water.
Granted this is a 'safety' switch, so
if you're concerned about relative safety, warranty, etc. This is not for you.
I am breathing life back into a system which currently is not being serviced here in the USA.
Perhaps another vendor will step up and fill in, but personally I don't see any sense in buying a replacement cel until there is a USA vendor readily available to service it.
Hope this helps some of you.
Happy Swimming.