hi guys ! I have a 99 Chevy Blazer 4.3 and the check engine light is on ! I had it checked and it came up 02 sensor on the drivers side ! so I replaced it, cleared it and after awhile the light came back on ! had it checked again and the same code came up ! why would the same code keep coming up after I replaced the sensor ? how many 02 sensors are there on my truck ? do I have to replace all of them ? it don't have any effect on the running of the truck ! it just shows up a E-VAP code ! thanks guys !

 

Your truck has 4 sensors. 2 on each side. What is the actual code you are getting? This will tell you which sensor is bad.