I had a random P0380 pop up a few times near the end of last month; which was strange because it was so warm that I'd think the glow plugs wouldn't even need to come on. I replaced the controller a couple years ago (forget the brand though) so I was going to be a bit upset if that was the problem. I noticed the nut on the constant 12v was loose so I tightened it down and did some testing just to be thorough. All values at the relay tested the same as at the battery (12.5v), but my sample glow plugs on each side tested 11.5v. I assume this is normal due to the large draw to heat the glow plugs but couldn't find anything in the manual indicating what the normal voltage range should be in the GP wiring, so I thought I'd put it out there. I plan on eventually replacing all 8 GPs but know it's quite an adventure to get the two passenger rear so am holding off till I know I have a full weekend free.