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I have a 2011 Chevy Traverse with a P2099 CEL that I can't get to go away. I used an OBD scanner to watch the live O2 and fuel trim data. The downstream O2 sensor on bank 2 (side that P2099 refers to) will register around 0.1 volts for a while, jump up to 0.6 volts for a while, and go back and forth. (NOT rapid fluctuation like the upstream O2 sensors register).

The long term fuel trim on this bank was -7% on this test drive. The saved info with the code showed the LTFT at -9.4%.

From my understanding a low voltage of 0.1v means it is lean. I also understand that a negative fuel trim value means the engine is trying to lean the fuel mixture. I also read that the computer looks at the downstream O2 sensor data to determine LTFT.

So I am confused. If my O2 sensor is registering lean, why is the computer trying to further lean the fuel mixture? Perhaps my underlying assumptions are incorrect, if so clarification would be appreciated.

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  • 1
    Have you checked your exhaust for leaks? A leak will introduce air into the exhaust system making the downstream O2 sensor appear more lean than it should be. Feb 28 at 8:11
  • Are you seeing similar fluctuations on the post cat sensor as well?
    – GdD
    Feb 28 at 10:06
  • @Pᴀᴜʟsᴛᴇʀ2 Yes my mechanic already looked for leaks, he said they found a tiny one on the joint right after the CAT and before the downstream O2 sensor. He fixed it. Still getting code and weird readings.
    – Mr. Spock
    Feb 28 at 14:48
  • @GdD The slowly fluctuating sensor I referred to is the post cat sensor bank 2. The post cat sensor on bank one (no code on that side) stays consistently around 0.6v which is what I would expect.
    – Mr. Spock
    Feb 28 at 14:49
  • 1
    Is Bank 1 Sensor 1 switching back and forth, ie 200-800 millivolts? Feb 28 at 17:30

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