I had the same problem, the heater would stay on when the sides were off, but put them back on and the same problem happened. For me, my gas pressure was too high, so try turning your gas valve half way off and see if the heater lights. That fixed it for me.

This could be a problem with your exaust or the sensor in it. You also have a negetave pressure sensor on this heater for cabnit air. This is wheren I would start then check all wire connections and your ignition module.