i have a 99 s-10 blazer 2wd 4dr LS
and i need to replace the o2 sesors.
the qestion is how many i have counted 3
but is there more?
2 in front of the cat and 1 at the cat

I show 3 - 02 sensors.
But always look and count them to be sure.
Always look in front of and behind cat.
One rear of cat and one on each side of front exhaust pipe.
Any problems with old ones or codes?
What is the mileage.
MT

yes 2 fromt and 1 back
and my ob2 gauge told me o2 sensor was bad
and i have 120000 miles
it also sad my mass flow sensor is bad
so im going on www.rockauto.com to order all these and see iff that fixes my surging problem.

my 96 4door LS has 4. just replaced them 3 wks ago. $225 for all.

2 upstream 2 downstream.

What was the code no.
Many things besides a bad 02 sensor can set a 02 sensor code.
Codes do not say replace part but say to check it out first.
MT