I test drove a 2005 2500 HD today with the Duramax engine. It has a tuner installed and some other upgrades. I don't know what the tuner is, it does not plug into the OBD II port. It has a 5 position rotary switch to select power level that's located underneath the steering column. The truck after it warms up Has a fishbite Miss occasionally at idle in gear only. The guy selling it is the original owner and said the injectors have never been touched or the harness. The check engine light is not on. He's going to take it to the shop that install the tuner years ago and have them see what they think. Any ideas? I gave him some money to hold it for me it is a nice truck.