Suburban 1993 2500 -454 / It runs rough on start up and then leans out and then the motor idles faster. It ran fine up until yesterday when my wife could not start it. Could it be a O2 sensor or fuel filter.it is a throttle body Carb.

By "throttle body carb" I am assuming you to mean that the engine is equipped with Throttle Body Injection (TBI). The O2 sensors on that engine do not even start working for about 5 minutes. They are not going to cause a problem on start-up. A fuel filter will most likely not cause a problem on start-up or idle either. A fuel filter is most likely to cause problems on acceleration or when pulling a hill. A more likely cause would be a n Engine Coolant Temperature (ECT) sensor. If one of these is reading too low, it will cause the engine to flood and smoke black out the exhaust - especially on start-up. (LIke a choke sticking on an old carbureted engine.) If your truck is equipped with a gauge, there may be 2 Coolant sensors. One for the gauge and one for the computer. If you replace it, make sure you are replacing the sensor for the computer.