# 2014 LTZ P0420 and intrmitent P0171



## Robby (Mar 1, 2013)

I suspect two, not related concerns.

Home in on the catalyst code for starters since it is hard, recurring code.
Helps to understand what the 02's are doing so you can wrap your arms around it.

On a inline engine with a single exhaust all codes are Bank 1......Bank 1 is always the side of a V type engine that #1 cylinder is located. So, on a GM V-8, bank 1 is the left (as sitting in the car looking forward) first cylinder. Same engine, bank 2 is the first cylinder on the passenger.
On a inline engine, since #1 is first in line, there is only a bank 1.

With that out of the way, the front 02.....the post catalyst sensor,(Bank 1 Sensor 1) sends information to the ecm and that information is shown as a continuous rich/lean, rich/lean command......very rapid, measured in milliseconds. On a reader you can watch the numbers rise and fall. The computer is always trying to make the perfect fuel mixture.....hence the rapid changes.

The post catalyst sensor (Bank 1 Sensor 2) is looking at the results of the catalyst treated exhaust.....on a reader, it will have a rather smooth rise and fall and there is a pre-determined value (change) expected pre-cat and post-cat.
A bad catalyst will be shown as a Bank 1 Sensor 2 telling the ecm that there is not enough difference between its readings and Sensor 1's readings.

In your situation, ideally you should have a true engine running diagnostic to see if both 02s are (focus on post cat) showing normal activity......I have had 02s 'Lock'......no activity for several seconds. That is not normal and will set the code described.

Through all this I have to ask.....are any other electronics doing wonky things? Like the radio shutting off or the instrument panel shutting off/recovering.
I mention this because of the negative battery cable special service policy....any voltage disruption from high cable resistance can stack odd codes.
This part has a warranty extension if the above weird things happen.

Regards,

Rob


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## gdcpony (May 29, 2017)

Robby said:


> I suspect two, not related concerns.
> 
> Home in on the catalyst code for starters since it is hard, recurring code.
> Helps to understand what the 02's are doing so you can wrap your arms around it.
> ...


No other electronic issues to note. The side sensors are unplugged due to being corroded, but the connector pins have been cleaned and sealed until I replace the sensors later. 

I have some screen shots of the live data while driving if that helps. I can't make real sense of the O2 readings on them. Lets see if I can load them. One is under load, one is coasting.















I did a "Mode 6" run on it which I guess tests the sensor and they both pass, but Being how new I am to interpreting these, I wouldn't trust that.


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## Robby (Mar 1, 2013)

Show me O2 information while idleing.......Coast shows fuel shut off....hence no readings (normal response).

Cruise readings don't help since these are load dependent and provide poor info.

Rob


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## gdcpony (May 29, 2017)

Robby said:


> Show me O2 information while idleing.......Coast shows fuel shut off....hence no readings (normal response).
> 
> Cruise readings don't help since these are load dependent and provide poor info.
> 
> Rob


I just saw this. Give me a little bit. Wifey is out with the car. Operating temp ok?


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## Robby (Mar 1, 2013)

I don't see operating temp on your screenshots....

Rob


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## gdcpony (May 29, 2017)

Sorry it took a while. I couldn't fit that data on the screen shot. Here are two more.







This is engine cold 2 min after start. 







This is engine warmed up after a 20 min drive. 

Hopefully this helps. Still learning to work with this scanner. 

I just did a plug change with no effect, same code. Doing an ETC and O2 unless I get a reply pointing elsewhere.


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## Robby (Mar 1, 2013)

Notice how your cold start O2 sensor 2 readings closely follow the sensor 1 readings? This is a cold catalyst not yet converting.
In open loop (cold) the sensor 1 calls for a rather rich fuel mix to aid driveability and promote a rapid catalyst 'fire up'
Those two readings, sensor 1 and sensor 2 are consistent cold engine readings are fine.

However, once in closed loop, it appears sensor 2 is dead and locked. Sensor 1 will show a relatively controlled wave as it is trying to hit that perfect, 14.7 to 1 fuel air ratio. Your number one appears to be responding normally.

Sensor 2 should be showing a rather gentle up and down wave......so when sensor 1 is rich (high wave) sensor 2 should also show a very small rise.......like .695 to .900 or even over 1.0......but the key here is gentle wave.
It appears to me that the post cat sensor is locking up when at operating temperature based only on the screenshots and that would be first on my part replacement list......and a locked post cat O2 will set efficiency codes since the info it is reporting back to the ecm is not at expected values.

A recommendation.....buy this from a dealer.....I've chased my tail endlessly with aftermarket parts.....seems to be no consistency from a quality standpoint.

Report back for the readers.

Rob


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