I have a Hayward H400FP propane heater that has not been used very often. When I turn it on the fan starts followed shortly by the propane igniting. It runs for less than a minute and then shuts off . When it shuts off it makes a noise that sounds like rocks rattling around inside the heater. I then get a code of HS or sometimes LO. I'm thinking there could be low water flow to the heater but all the valves are correctly set and the cartridge filter cleaned. Any ideas? Thanks !