A 2013 Cummins in Grand Rapids taught me to check the cheap stuff before pulling the valve cover
Truck came in on a Tuesday with a misfire and a $1,800 quote from another shop for injectors, so I checked the fuel filter first and found it was the original one from 2013 with 190k miles on it, and a $22 filter fixed the whole thing, so what is the most expensive misdiagnosis you have watched walk out your bay?