What to do with pre cat and after cat 02 sensors

[supspt454]

97 GMC Sierra factory 5.0L exhaust uses a small down pipes with O2 sensors in each bank, then into a Y pipe with a pre cat O2 and after cat O2 for a total of 4 sensors. Truck has a 383 with mid length headers, 3" to 2.5" header collectors with O2 bungs for each bank. Head pipes will be 2.5" duals into a Jones Full Bore 2.5" dual in/out with dual 2.5" tail pipes. I have a 411 PMC to swap and I have HP Tuners that I have not used yet, so there will be a learning curve. I understand that the pre cat sensor in the Y pipe is to read both banks, and after cat is to read if its doing its job. Is the pre cat sensor actually needed since it has one for each bank? How can I disable the reading in HP Tuners for those two sensors? Trying to learn. Thanks.

 

[supspt454]

From what I have found, bank 1 sensor 2 and bank 1 sensor 3 are what I am asking about for the y pipe into single cat exhaust that the 5.0L had from factory.

 

[b454rat]

You need the pre cat ones, not the after cat ones. They just read the condition of the cats.

 

[supspt454]

You need the pre cat ones, not the after cat ones. They just read the condition of the cats.

So I need all 3 pre cat sensors? If so, where do I put B1S2 if it had 1 factory cat? Just in one of the pipes of the duals, which would mean that pass bank would have two sensors and driver bank would only have one.

 

[b454rat]

3 pre cat? Don't think I ever heard/seen 3 pre cats. I had a 96 with a 305, had 2 pre and a single post O2.

 

[L31MaxExpress]

3 pre cat? Don't think I ever heard/seen 3 pre cats. I had a 96 with a 305, had 2 pre and a single post O2.

Nope it had a 3rd pre-cat. All of the 4.3L and 5.0L Vortecs did.

 

[Orpedcrow]

My 5.0 truck has 3. Driver side is about oil filter level, passenger side is at the end of the tube before the merge then another in the merge in front of the cat.

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[Nad_Yvalhosert]

That's right. The single catalyst Vortec smaller engines do have 3 upstream sensors that sample left, right and combined pre catalyst. Then a single downstream post cat sensor.

I was made aware a bit ago, as I thought all Vortecs were dual pipes, dual catalyst, into a single muffler and tailpipe.

@supspt454 have your tuner remove the 3rd O2 sensor PID in the PCM. Use 2 up and 1 down sensors. Then cover the pigtail, maybe even cut the sensor side wires and cap them off to protect the pins in the pigtail.

 

[99xcss4]

that is weird I have only seen 1 305 vortec truck it was a 99 gmc K-1500 classic it only had 3 o2 sensors on it 2 up stream and 1 post cat

 

[L31MaxExpress]

That's right. The single catalyst Vortec smaller engines do have 3 upstream sensors that sample left, right and combined pre catalyst. Then a single downstream post cat sensor.

I was made aware a bit ago, as I thought all Vortecs were dual pipes, dual catalyst, into a single muffler and tailpipe.

@supspt454 have your tuner remove the 3rd O2 sensor PID in the PCM. Use 2 up and 1 down sensors. Then cover the pigtail, maybe even cut the sensor side wires and cap them off to protect the pins in the pigtail.

That did change in the GMT800 4.3Ls, they had dual cats with an 02 in front and behind each. The GMT800 5.7L was catless with front 02s only.