After looking up youtube videos on how to correct the "door open" error message I replaced the blown thermal fuse on the heater housing. I cleaned the exhaust hose and filter to like new condition. I even tested the cycling of the heat going from high heat to step down after 15 minutes with a thermometer to be sure it was not running on high heat 100% of the time (youtube video).
The thermal fuse kept blowing. I found appliancepartpros on the internet and called tech support.
The very helpful technician/parts expert advised to replace 2 other parts associated with this common problem (high limit thermostat and second thermal fuse on exhaust). I watched the videos on the website on how to install and it was very simple. I feel pretty confidant that this is going to fix the problem for good! Thank you!