30,000 btu kerosene ready heater

I have a ready heater that is pretty much automatic. Plug and play if you will.
it started to go bad a week or so ago and i thought i could solve the problem myself. Thinking logically, first i tested the polarity of the igniter and no problems found. I then cleaned and replaced the fuel nozzle, same problem.
Lastly i replaced the photocell and still can't keep it going.
Heater lights right away but goes out after about 20 seconds or so.
Guess I'm not a heater mech after all. Any suggestions will be appreciated. Thanks in advance.