Diesels don't have throttle bodies.. it'll be obvious which plug is bad. it will read an OL or a high resistance that are different than the rest. Remove the connecter from the relay and check them on the ohm settings on your meter.

after that clear the code and see what happens

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k