^^^ What he said.
Pull the RH inner splash shield out, and you can see it pretty easily. Heat the bung up in the pipe, hit it with some GOOD penetrating oil, and wind it out. If it fights you, just keep heating it gently until it moves freely. You ABSOLUTELY DO NOT want to haul the threads out of that bung -- up-pipe replacement is likely necessary afterwards (you CAN re-tap the bung, but I haven't had a great deal of luck with that process, usually it completely destroys the threads on the way out...I live in the salt belt) and that is not very much fun.
The P1635 is likely because of bigger tires. Can also be from the wrong axle ratio programmed into the PCM, or a corrupted VID block in the PCM. But I'd put my money on the bigger tires.
Hopefully both of our replies have been of some use to you.
If you watch your EGRTA PID on your Spartan, I would bet that it's jumping all over the place, OR staying at 140 degrees, OR at -40 degrees.