AR154U

I have a easy life spa with a Gecko heater and I get a code of RHNF. I checked all the pumps and there is no air locks. Then I shut down all the power for 20 min turn it all back on and it will start the circulating pump and run for about 10 min and then start flashing RHNF.I was told that there was no water going though the heater so I took the load side water piping off the heater and yea there is plenty of water going thought the heater.Now I was told the heater is shot REALLY???? So could there be any other thing wrong a heater is 350.00 bucks the heater model number is 0603-421002 Thanks for any help with this