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Truck is running too fat. Might never get to "closed loop" and running full rich. Get a scanner to pull the code. Might go to local parts store, they will pull the code, and sell you a set of sensors. Easy to install, they will even have the O2 sensor socket. Sometimes the sensor goes bad because it does not ever get to temp. and it thinks it is still cold, so it goes full rich, heak any number of variables can cause this, but the first thing to do is check the code, it is there to tell you what is wrong.