the best way to figure out what is wrong is to hook up the diagnostic pc (diagnozer) program and see whats going on.
Otherwise if there is anything wrong it should be showing an error code on the display in the hour meter LCD window.
The LP interlock is actually part of the vaporizer and since that was replaced i doubt that's what is wrong.
You can look at the LP injector, pull it out and see if it is oily and dirty. Also check the inline fuse link in the harness going to it, there is a small black square fuse holder with a minifuse in it on the side of the injector body.
What is the serial number of the lift?