Hummer fuel injectors need to be a very tough breed to survive the rough handling that Hummer drivers mete out on their vehicles. Typically, the Hummer fuel injectors will start to fail because of shorted or open circuits leading to the sensors and the on-board computer that regulates the engine functions. Your Hummer fuel injectors depend on those circuits, which regulate when, how often and how long the fuel injector nozzles will remain open. Those nozzles release the fuel into the cylinders at highly regulated volume and pressure levels. When running properly, the fuel and oxygen levels will be measured out according to the engine's needs. When the air-fuel mixture fails to reach proper levels, you will notice engine detonations or misfires, knocking, loss of power and a drop in your gas mileage. Detonations will eventually jar your engine so badly that damage might occur there. There is the risk of failing an emissions test because of the high levels of hydrocarbon and carbon monoxide emissions. These high levels come as a result of unburned fuel passing straight to your exhaust system. Some of the tests you can use to diagnose Hummer fuel injectors include flow testing, which compares the actual flow rate of each injector to factory specifications. If these tests pinpoint the issue in your combustion system comes from the injectors, order your new Hummer fuel injectors here and replace them yourself.