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Discussion Starter #1
Hi everyone,

I own a 2004 Sentra 1.8. I recently bought an OBD-II scanner, thought it would be a neat gadget to have, used it on the car and realized that my engine runs mostly in "Open Loop - System Fault" mode. It occasionally goes into "Closed Loop" mode but only briefly. Can anyone tell me what could be the cause and help me find the problem?

Thank you.
 

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Most likely it's caused by a bad O2 sensor or it's connecting circuitry.

Also during idle or if the engine is not fully warmed, the system will remain in "open loop".
 

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Discussion Starter #3 (Edited)
I took a trip and logged the data. It's mostly open loop. I also tried to see what happens if I unplug the O2 sensor. The data is totally different, so there's definitely some feedback from it but this doesn't necessarily prove the sensor is good. Is there a way to test it?
 

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I took a trip and logged the data. It's mostly open loop. I also tried to see what happens if I unplug the O2 sensor. The data is totally different, so there's definitely some feedback from it but this doesn't necessarily prove the sensor is good. Is there a way to test it?
Is the check engine light on? If there was a problem you would get a code for closed loop. The system will stay in open loop untill the engine is at operating temperature and then go to closed loop. open loop allows the engine to run on preset values when it is cold. When it switches to closed loop it will manage the engine operation by adjusting to the sensor inputs.
 

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Discussion Starter #5
Hey mpe235. Thanks for the reply.
Just one remark regarding you reply: you got the open-loop closed-loop terms mixed up. It's actually the other way around.
The engine was definitely at normal operating temperature. The check engine light is not on, no trouble codes.
 

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Hey mpe235. Thanks for the reply.
Just one remark regarding you reply: you got the open-loop closed-loop terms mixed up. It's actually the other way around.
The engine was definitely at normal operating temperature. The check engine light is not on, no trouble codes.
You are correct. I edited the previous post to correct it. Thanks
 

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Discussion Starter #8
At this point I'm fairly convinced that the upstream O2 sensor is bad. I did another run and it's reading is much to stable at around 350mV. It does spike up once in a while, but very rarely. I wonder why it does spike though. Anyway I ordered a new one which I'm expecting mid or late next week. I'll let you know if that will fix my problem.
 

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Discussion Starter #9
Well, I had the sensor replaced and all I got out of it is a new sensor. The engine runs the same way as before, so the problem is somewhere else, maybe in the harness. I'll have to do some more investigations.
 
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