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My 17 3500 srw had same problem. New gas cap didn't fix it. Finally dealer had time to look into problem(after waiting 2 months to get a time slot). Smoke test sent them to top of tank. After dropping the 52 gallon tank, they found that "critters" had chewed through the plastic line that runs to front of truck for EVAP gasses return. I did not get to see the evidence, they had truck for a whole week, would not warranty repair cause of "critters". Cost me $430 and a week of lost wages. I make my living with that utility bed.
This was the issue with mine recently. Went by Auto Zone, they pulled the code and printed off the likely issue which was the ESIM and that solved it. Part was more like $50 where I'm at but still less than anyone would have charged to fix it. Took about 2 minutes to swap out and that's because I took my time. FYI if it is the ESIM it may take a few days for the check engine light to go off. Took my truck 2 days before it cleared after I replaced the ESIM.Get a new ESIM, they're like $20 and easy to put on. 90% of evap problems on these trucks have to do with the ESIM
Same thing with my last van, a few days after I bought it new. Chewed up the venting tubing. Thing is the check engine light does not show a problem until several start and stop cycles take place else it would have been on during the drive home from the dealer.My 17 3500 srw had same problem. New gas cap didn't fix it. Finally dealer had time to look into problem(after waiting 2 months to get a time slot). Smoke test sent them to top of tank. After dropping the 52 gallon tank, they found that "critters" had chewed through the plastic line that runs to front of truck for EVAP gasses return. I did not get to see the evidence, they had truck for a whole week, would not warranty repair cause of "critters". Cost me $430 and a week of lost wages. I make my living with that utility bed.