Sometimes these trucks will throw an oxygen sensor code if the fuel mixture cannot be regulated. Replacing the oxygen sensor in these cases will not clear the code. You need to check the mixer (carb) for excessive wear, check the idle mixture screw to ensure it is at the correct mixture (need an emissions tester to dial it in), and if the fuel control solenoid switch (a vacuum solenoid that connects engine vacuum to the ambient pressure vent on the vaporiser) is malfunctioning or has a leaking vac. hose, it will also throw this code. Call a technician who has the proper testing equipment and knowledge to do this.