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Started to get a running rough about 2 weeks ago. Fuel trim in negative. Replaced both downstream O2s. Anyone have any clues? I would greatly appreciate the help.
 

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Yes! I had a similar problem on my wife's CTS, although that problem was too rich as opposed to too lean.

More than likely NOT the O2 sensors - they're likely telling the truth (save the old ones, they're probably still just fine).

Likely causes:
* High pressure fuel pump (on the CTS this was the cause - the one on the engine, not the main pump in the tank)
* Fuel pressure regulator
* Main fuel pump in tank (low pressure)
* Dirty/malfunctioning MAF sensor - disconnect it and see if it gets better

(there are probably other causes - those are some of the more common ones)

Something I found helpful was this specific scan tool:
https://www.amazon.com/BlueDriver-Bluetooth-Professional-iPhone-Android/dp/B00652G4TS/ref=sr_1_4?s=automotive&ie=UTF8&qid=1536679803&sr=1-4&keywords=bluedriver

Yes, it's pricier than a lot of them but this thing is professional level and can show you *all* of the measurements your sensors and ECU are dealing with in real time, including things like current fuel rail pressure (which might be helpful in your case). It can also record history and show you graphs of all these readings over time.

The main challenge for me was finding accurate and current information about what the *normal ranges* are for all the various sensor readings.

Good luck,

Bear
 
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