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Q: Constant P0140 O2 sensor code on 2001 Nissan Sentra

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I have a 2001 Nissan Sentra that keeps showing a P0140 DTC. I was intitially getting a code for the heater circuit so I replaced the sensor. Thats when I started getting P0140 instead. I checked wiring for shorts and continuity to the ECM as stated in the factory service manual. Everything checked out good so I replaced the sensor again and have the same results. The code states bank 1 however the factory service manual states bank 2. in the end I ended up replacing all 4 sensors since one by one I was getting codes. All DCT's cleared up after replacing except this P0140 which keeps coming up. On the scan tool the sensor reads a constant 1.275 volts. I have checked with 2 different scan tools with the same results. I also checked all grounds and cleaned and tightened them as well. I don't know what else to try.

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Defective aftermarket sensor, post cat! NGK/NTK, DENSO brand work well. Circuit fault to/from sensor can be checked with DVOM. Oh yeah... Only one bank on a 4 cylinder! That's bank one...
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where are u buying sensor? iv seen aftermarket sensors cause a number of issues incl car runnabilty issues
Purchased from pep boys. I bought Bosche sensors since they were the same brand that came in the car. Also made sure they had factory plugs already. I checked circuit with DVM and all looks good. Everything pointed to bad sensor so I replaced it a second time with same results.
5 volts reference and ground at the 4 wire plug key on? 12 volts and ground for the heater cifcuit also at the 4 wire plug? Another way to check the 12 v heater citcuit is turn key on with sensor pluged in should get hot holding in hand! The sensor BEHIND the converter! Any blown fuses? Check ALL!
OK, I had a chance to check out your suggestions. I have confirmed 12 volts and ground. I verified heater current however there is no 5 volts reference. I checked other downstream o2 sensor which is working normal and got the same identical readings on both plugs. I did notice for some reason my bank 1 fuel trims especially short term fuel trim was high around 20%.Bank 1 upstream also seems to stay more around the .05 volt readings but does fluctuate between 0 and approx .8 volts while operating the throttle and seems to function normally while driving. Also I noticed the ECT on scantool does not rise above 170 degrees while driving. I should also mention I checked for reference voltage with key on engine off. Not sure if that is the correct way. No blown fuses either. This is so frustrating. The car runs very good but can't get through emissions. It does idle maybe just a little rough after it warms up and idles relativly rough when ac compressor kicks on.

Hey, did you finally resolve the problem? My 2001 Nissan Sentra also has the same problem - tried both Denso and Bosch sensors, but p140 keeps coming!! and the Bank1 Sensor 2 voltage is constant at 1.275.
Yes I did. The issue was a bad ECM. I verified that the O2 sensor was working by monitoring the signal at the ECM connector.
Thanks for your reply! I think that's the problem with my car too. Did you buy a new ECM or you got it repaired? Do you know where the ECM is located in the car?
It's under the hood on the passenger side firewall. I bought a new computer. Cost like 500.00. Most used ones I found were around 400.00
Thanks!! yeah, I found it, took it out and verified continuity, but when I put it back, now I'm getting 4 new codes - all pointing to "o2 heater circuit"!! Did the fuse blow up? Resetting the code didn't fix it. I was planning to take my ECM out and get it repaired (they charge $85), but now the new codes have created another head ache. Any idea what could have gone wrong?
Its been awhile since I looked at the schematics but if I remember correctly the ground for the heater circuit is applied through the ECM. First thing I would suspect is a bad connection at the ECM. The factory service manual really helped me here with the troubleshooting! I was able to download it for free.
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