How are you getting the code? Like how are you reading it i mean? There was a problem back in the day with this code popping up as a random with certain canned tuners, but not displaying on a V2/ diagnostic machine. You are correct on the #1 glow plug.
Anywho, to test a glow plug, take the rail off of them and get a test light, hook it to the pos side of the battery and touch the tip of each glowplug. if it lights up the glowplug is bad