My CEL came on for #2 and #3 glow plug, I don't remember the specific code # but I saved it on the computer just in case. I figured while I was replacing those 2 I might as well replace the other two so I wouldn't have to worry about them failing later. The Jetta has 260,000 miles on it so I think it's fair to say that the GP's have had a good long life! I noticed the harness going to them, the thick plastic insulator was cracked in some places so I'll check that out too and see if it needs to be replaced as well.