My 550 is getting a little worse when it's cold. Always starts right up, but whole it used to only stutter and miss when it was close to 0F, it seems to do it now under 20F. After a bit of internet sleuthing, it appears that the check engine light should be on if I have any bad plugs. It does have a 50 hp tune that was installed when a couple parts fell off, could the tune disable some of the functionality of the CEL? I'm slightly worried about a bigger problem, one that I don't want to mention for fear of jinxing it.
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Bad glow plugs will not set a Cell. You would have to remove them and test each one. A bad glow plug controller will however set a cell.
Might be a good idea to replace glow plugs every 100k I didn't really think the 6.7 needs glow plugs though. My 6.0 sets a cel for individual plugs, I have one right now for it
Beru are the good ones.