My blazer is OBD1 and it only has one, four wired O2 sensor in the Y-pipe.

 

I can only find 2 but I keep hearing rumors that there may be a third that has to be replaced as well. I have one on my Y-pipe (already replaced) and another just behind the cat and right before the muffler. I am going to replace the one between the cat and the muffler and see what happens

 

If you had a third it would be located on the drivers side y-pipe just below the y-pipe to exhaust manifold connection, somewhere around the frame rail.

 

Try this

 

Fog, that picture is for the 98+ trucks.

A '95 OBD1 setup will only have 1 O2 sensor just after the 'y' in the y-pipe before the catalytic converter called the upstream sensor. The OBD1.5 & OBD2 '95 models will have 2, same location as the OBD1 setup as well as a post cat sensor. NEVER replace a post cat sensor unless it's setting a code and even then, only after properly diagnosing the code to determine if the sensor is actually faulty. Only the upstream sensor has anything to do with fuel management in the engine.

The sensors themselves are identical in every way; the AC Delco # is AFS75.

 

Alright. Guess its time to get the code now and see where the issue is. At least there is only 2 sensors on my truck, thanks guys for telling me. Do you know of any places that can scan an OBD1.5 code off a 16 pin connector?