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Diagnostic Tool help (02 sensor analysis)

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I recently have been having more rough idle on my car and was wondering if it might be the 02 sensors. For a while, in the summer, when the car had been driven for a while, starting it up on first try or 2 the engine would crank but then immediately die, and then if I held the gas down a little while cranking, I could get it working. I cleaned out the throttle body, air filter, and MAF recently, but that didn't change the problem. After changing transmission fluid recently, the car ran really rough immediately after, not wanting to stay cranked unless I constantly pumped the gas, but then after about 5 min of that, the car stabilized and hasn't had that symptom anymore.

I've attached a video of my diagnostic tool while the car is sitting at idle to see if anyone sees anything abnormal with the 02 sensors. I am not familiar with looking at this reading and being able to tell which 02 are the pre-cat and which ones are the post-cat. I'm at 90k miles. Any thoughts?



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It's a 1999 Carrera. I just checked the link and it worked for me. You might have to right click on the link and choose "Save link" or "Save target" to download the video. Otherwise, it probably just waits until the whole file has downloaded before playing back.


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Ok, saw the video now.

S11 = bank 1 precat

S12 = bank 1 postcat

S21 = bank 2 precat

S22 = bank 2 postcat

Precat should swing between 0.1 to ~0.7v about every second at idle so yours look fine.

Postcat should read about 0.7 solid most of the time when cruising. It will go high when you give gas and low to 0 when you decelerate.

Have you cleaned your idle stabilizer? Also check the purge valve beside the throttle body. I doubt the rough idle is due to O2 sensors.

Edited by Ahsai
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Hey Ahsai, I am not sure of the difference between those two. I did see the "idle control valve" to the left of the throttle body when I was taking that stuff out, but was afraid to undo it since I didn't have a new, spare gasket to replace it with if the gasket was worn once removed. Where is the purge value?

I looked for a tutorial on cleaning the idle control valve, but couldn't find one. Do you know of one or could you explain to me what I need to do?

Thank you for helping me with this.


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