My 2014 GMC 3500HD Diesel truck (44K miles) just popped its first CEL light. Took it to an auto parts store and they read "DTC Error....undefined (Permanent)". They cleared the light but it came back in a couple of days. Seems a bit suspicious that the light came on within a few miles of the dealer sticker showing next service. 5000 miles after I had the last service. Is it possible for the dealer to set up a CEL when you approach the suggested service mileage so you'll be paranoid enough to bring it in? Manual says 7500 per oil change. First experience with this dealer. One extra bit of info....my dear wife took the truck on a 1800 mile trip with it in 4WD. Did she do any damage? Seems to still work fine.

 

no dealer has a way to time bomb fault codes...yes, you are paranoid.

there is a fault that has set that the auto parts scan tool could not define. have it checked at the dealer where they have the proper equipment.

driving it in 4wd on dry pavement will stress the tcase during turns when each wheel must turn at different speeds...if its working fine, don't worry about it.

 

Just an update. Dealer found a defective DEF tank heater, which popped up the CEL light. Fixed at no charge (still in warranty). I apologize for the paranoia. Not all dealers are crooks.....guess I've just had some bad experiences.