Postby Lit » Thu Feb 22, 2018 11:56 am
I agree with real2k1. I had the same problem due to the trans fluid radiator bypass that would cause my 2008 Frontier to throw that code, limp mode, when towing in 4Lo. Note, the V6 Pathfinder and Frontier have the same 4.0 engine, radiator and trans.
Every time when I would tow my 3800lbs camp trailer to a certain camp spot that required about 1/2 mile in 4Lo, the trans would go into limp mode and I would get the U1000 code. I would be able to make it the last short distance by turning off the truck for a couple of minutes and then restarting it. The next morning it would have cooled down and I could not reproduce the problem. The tow home was all downhill, so I the problem would not come back until next time I was towing to that camping spot. I did not have a Trans temp gauge on the Frontier, so one time I brought my IR temp gun to check the Trans pan temp when it went into limp mode and it was at 245 degrees. I do realize that was not really an accurate test of the temperature of the trans fluid, as the engine, exhaust and catalytic converters would likely effect that temp, but either way, I think it was way too high.
I only did the bypass to prevent the SMOD problem. So I ended up undoing the bypass and replacing my radiator and the problem has not returned.