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

RBALLZ2

MEMBER
2003 FORD F-250
5.4L
V8
4WD
AUTOMATIC
230,000 MILES

Engine is Throwing a code that says the number 2 (downstream) oxygen sensor is faulty but both sensors are up on the exhaust by headers. I replaced them both 3 to 4 years ago. Is that normal for them to go out after 3 years? Also, which would be considered the downstream O2 sensor?

5 Replies

JACOBANDNICKOLAS

MECHANIC
109,724 POSTS

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.

Image (Click to make bigger)

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Tuesday, September 12th, 2023 AT 6:40 PM

RBALLZ2

MEMBER
3 POSTS

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.

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Tuesday, September 12th, 2023 AT 7:19 PM

JACOBANDNICKOLAS

MECHANIC
109,724 POSTS

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

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Wednesday, September 13th, 2023 AT 5:44 PM

RBALLZ2

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3 POSTS

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

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Wednesday, September 13th, 2023 AT 5:55 PM

JACOBANDNICKOLAS

MECHANIC
109,724 POSTS

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.

Image (Click to make bigger)