'99PSD just replace all glow plugs, the GPR checks ok, won't start in this cold temp(mid 20's) unless I plug it in. Code 603 just started showing up as of late. What is it telling me?

 

The 0603 is caused by your tuner, is perfectly normal to pop up when switching tunes, and has nothing to do with your cold start issue.

How did you go about diagnosing the GPR? If you can't start it unless it's plugged in, then it's definately a problem with the glow plug system. If you've already replaced the plugs, then the GPR is the next culprit.

 

Whats the battery voltage during cranking?

 

A couple of question! Why is ithe 603 code just now showing up? One new battery was just put in last month. Also this truck has 740000 miles on it, untouched.

 

from what i have read you should always replace both batteries at the same time cause if one is weaker then the other it will drain the new one down. Try and measure the voltage when you try starting it again. I would say batteries or gpr if you already changed the gps and it will start when you plug it in but not when it's cold and not plugged in.