I have already searched and found alot of information on this SES code. My question is when these guys are saying switch the two downstream 02 sensors, if the code follows then its the sensor. If not it is something else. What do you mean by if the code follows the sensor? Will it show the same P0420 code after the switch? If it shows P0430 then the sensor is working properly? Sorry for the dumb question, kinda new to working on my own vehicle. Any explanations would be greatly appreciated. Bye the way it is a 1999 Silverado 4.3 4x4 5 speed manual. Also, the truck is missing randomly when I am accelerating.