Transmission oil change price?
$60 is actually a really good price. I'm pricing a transmission flush/service for my 2002 VW Golf and the best price found is $89.95 (and this is with a $35 off coupon). Make sure that whoever is doing it is doing a FUL fluid change (including the torque converter etc.) and replacing your gasket and filter. (you're referring to transmission so I assume you're talking about your ATF fluid NOT a standard oil change)
It does sound like it has restriction in the inlet and exhaust lines. The installation manual will specify what the total run (intake AND exhaust together) can be, with how many 90s, 45s, etc. If the total is too restrictive, it can have firing problems like that. Taking the plug out removes enough restriction so it will fire. That's a good test. It needs both to be unrestricted to work right, if either one, or the combination are too restricted, it won't work right. 3 is pretty good, but check the specs in the manual, add everything up and see if it was put in right. I don't think the gas valve is the answer. The best reason for pulling the air in from outside is that it's not pulling already heated air out of the house and wasting it, and also because when you pull the air out of the house it puts the house under negative pressure.that air has to come from somewhere, and it will come in around your windows, doors, outlets, etc. adding to the drafty feeling.
I have had these installed in some of my construction projects. Most brands can be installed with interior air or outside air for combustion make up air. Theoretically the use of outside air is more energy efficient in that the negative pressure created by the combustion process causes air infiltration from leaky areas like windows and doors. As far as affecting your warranty I would read the installation manual and make certain you are not voiding it. The week point for these furnaces are the circuit boards that act up and the condensation tubing. If the drain line clogs or freezes your furnace will not open the gas valve and then you will not have heat.
18 of 3 pipe is too long. The longer the run, the more restriction on the flow of air. It needs to be run in 4 and reduced to three as it enters the furnace. If the 3 run was 10 or less, you would be ok. You need to go to 4 supply piping for the combustion air. You said it works fine when you take off the pvc at the furnace. That is because you removed the restriction of air flow and it is using the air from the room you are in for combustion. Change it to 4.