Take a look under your truck. I cannot remember if it was 97 or 98 when they eliminated the B1S2 position, but I think it was 98. If that is the case, you will have two upstream sensors which are the only sensors that impact fuel mileage. These sensors are B1S1 and B2S1 where B=bank and S=sensor. Both of these sensors will be between their respective exhaust manifolds and the 'Y' in the y-pipe.

You then will have B1S2 which will be directly ahead of the catalytic converter and B1S3 directly behind the catalytic converter. These two sensors should not be replaced unless a code is triggered and they are found to be at fault after proper diagnostic steps were taken.

 

You then will have B1S2 which will be directly ahead of the catalytic converter and B1S3 directly behind the catalytic converter. These two sensors should not be replaced unless a code is triggered and they are found to be at fault after proper diagnostic steps were taken.[/quote]

Ok, I've checked and got 3 upstream O² sensors, but I don't need to replace the one directly in front of the cat, just the one on either side before the Y-pipe intersects correct? Damn I wish I'd have checked this again, coulda saved $60.... Last question, since I've got all my parts and ready to go...On this 97 Blazer, does the battery need disconnected for the computer to 'reset' and 're-learn' after replacing the sensors like my S-10 did?

 

Yes, correct. The only two O2 sensors on the 96+ trucks that have anything to do with the fuel delivery system are the upstream sensors which are between the engine and the 'y' in the y-pipe.

You can quicken the process by disconnecting the battery.

 

knew a guy that blew the electrode right off the spark plug using bosch . stickin with ac/delco rapid fire