I have an older model 155, bought in the fall of 2000 and at one time I had that same problem. I believe at that time I wound up replacing the filter on the end of the fuel pick-up line. I bought the filter(s) at a local rental/repair place. The only other thing I have ever had to do to this heater is replace the igniter a couple of years after the filter problem.

Hi all sounds like your problem is the fan is stuck.I had same problem I oiled up the fan and it works fine.You will now by trying to spin the fan blade.worst case may need a fan motor.Hope this helps.

Since you did not state anything about the presence of a flame, it sounds as if there is a fuel delivery problem. The Photocell is shutting the heater down because it does not see a flame.
Troubleshooting tips http://www.reddyparts.com/HSI_tips.htm
How to set the air pump pressure http://www.reddyparts.com/pressure.htm

You can refer to this manual: http://www.bealsmotor.com/Desa%20Hot%20Surface%20Manual.pdf
They have a detailed troubleshooting guide. You will need a multimeter and some electronic experience for some of the tests. I hope this helps!