Fix Propane Pool Heater Teledyne Laars Lite LLD 250?

[jemshkoj]

The heater pilot light will light but the main gas flow won't go through, except for just one time when i first tried it
I am using 20 pound tank and don't know if that matters?
Any help would be appreciated
thanks
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[Burnerman]

Keep the gas push valve held in for 20 sec once the pilot has lit.....if it goes out once released then you need a new thermocouple. Last one I did was around £12.
John