I threw a code indicating that the passenger front O2 sensor was bad. I replaced it, along with the drivers front and one behind the cat. The parts place, and everything I could find online said there were 2 upstream and 2 behind the cat. I can't find the 2nd downstream sensor. Is it possible that there are only 3, or where is the 4th?

Thank you so much in advance.

 

What year is your Blazer? That would help...

 

The year would help. I've never heard of two downstream O2 sensors, only one. If there are four, it's probably one in or near each banks exhaust manifold, and one before the cat, and one after. The downstream one does nothing for engine management, it simply monitors the condition of the catalytic convertor, I believe.

On my '95, I only have two, after the "y" pipe comes together before the cat, and one after the cat.

 

I'm pretty sure there are only 3. I know my 02 only has 3.

 

My 05 has only 3.

 

Sorry guys. I put in just over 90 hours at work last week. It's an 04 2wd 4 door. I have replaced all three. I have come to the conclusion that's all there are.

 

An 04 only has 3.

 

1995 and earlier only have 2 sensors, (not OBDII compliant). 1996 & 1997 have 4 sensors. On 1998 and newer, B1S2 was eliminated.

 

My 1st Gen 89 Blazer has only one.

 

Ahh, good call! Don't see many TBI's on the road around here any more, they're resting peacefully in the graveyard I should have specified "W" engines.