Service engine light came on,on a 96 blazer, and I was able to pull these codes. I have replaced the oxygen sensors if front of and directly behind the cat converter. Reset the computer and the engine light comes back on. I have noticed no change in the vehicles performance in fact my mileage went up after the engine light went on. I see 2 more o2 sensors, one on each header prior to cat converter. Do any of these need to be replaced? Am I Missing something easy? The wiring leading to the o2 sensor pigtail appeared intact when I changed them. Any ideas would be greatly appreciated.

I am having the same problem (same codes) with my 96 Blazer. Did you ever get it resolved? Which o2 sensors did you finally replace?

Your mileage probably went up because the computer went into "limp home" mode where it uses a preprogrammed set of fuel maps, instead of relying on the O2 sensors for exhaust feedback. If you have those codes I would recommend replacing the Upstream sensors as a pair and then reset the computer. Start with that and see if anything improves. -Chris

Through those codes you have a bad passenger side manifold O2 sensor.

Replace, reset your computer by removing the negative battery cable for 5 minutes and enjoy your truck back to normal.