Q: My Hummer shows code P0300 after getting gas

You may have got contaminated fuel from the gas station. The code P0300 indicates you are getting a random engine misfire. This random misfire does not point to any one cylinder misfire and indicates that the fuel may be contaminated with something like water. All gas stations have some water that is in their tanks in the ground. The water normally sits in the bottom of the tank and if it becomes excessive then the gas station will need to have it removed. Now if there is about 2 inches of water in the tank settled on the bottom of the tank and the refueling truck dumped new fuel in the ground tank then water will temporarily mix with the new fuel for about an hour. If you happen to get fuel at the time the water is mixed up in the tank then you would get some in your tank. This water will cause erratic running and misfires in the engine. If you go to the parts store and get some dry gas additive it will disperse the water so you can run it through the engine and as soon as you get to half tank then refill tank again to disperse the water more until the engine starts running better. Then you can get the Check Engine Light cleared.