Oxygen sensors, which would be considered the downstream O2 sensor?

Hi,

With an aftermarket oxygen sensor, they can fail a lot sooner than the original ones.

As far as the "downstream" sensor, it should be located after the catalytic converters.

I attached a picture below specific to the Ford V8 in your truck. I suspect it came originally with single exhaust. When you look at the pic, each sensor is identified as HO2S and followed by 11, 12. and 21. These numbers indicate the bank and sensor location. For example, HO2S 11 is the heated oxygen sensor bank 1 sensor 1.

Let me know if this helps. Also, if you have the code, I can check it for you to see specifically what it indicates.

Take care,

Joe

See pic below.

Thank you for the quick response. That's where I'm confused though. I have stock exhaust and only have the 2 upper ones. There's nothing down by the catalytic converter. Everywhere I look it says there's 3 but there's only the 2. I'm the original owner and have no modifications done and have never had any issues. I can't pass emissions, so I guess I'll try replacing them again, I don't know what else to do at this point.

Hi,

That's interesting. What is the code you have? That will point me in the right direction. Also, are you sure it isn't right after the converter?

Let me know.

Joe

P0156 is the code and I've looked over and over thinking I missed it, but there's only the 2.

Hi,

The code is for the HO2S B1S2. The only thing I can tell you is it is behind the three-way catalytic converter mounted at the top of the pipe. See pic below. If it isn't there, make sure it hasn't been broken or removed.

Let me know.

Joe

See pic below.