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05 altima 2.5 incorrect oxygen sensor readings
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jrittenb
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Jan 22, 2013, 6:56 AM
Post #1 of 8
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05 altima 2.5 incorrect oxygen sensor readings
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I got a P0420 code on my Altima so I began looking at my O2 sensors with a diagnostic tool that showed me the waveforms and voltages. The upsteam sensor was reading a pretty steady 0.4v with no oscillation The downstream sensor was reading a pretty steady 0.8 volts but jumping around somewhat while driving. Since the upstream sensor was not "switching" I decided to begin by replacing that sensor. The results were the same waveform. The steady 0.4v was because the upstream sensor was a wideband sensor. That answered that questoion but I cannot figure out how the downstream voltage reading could be higher than the upstream reading. This indicates there is more oxygen in the exhaust gas before the converter. Any ideas?
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jrittenb
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Jan 22, 2013, 9:51 AM
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Re: 05 altima 2.5 incorrect oxygen sensor readings
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Yes, I have read about this TSB and will probably end up getting the PCM reprogrammed to address the P0420 code.
My real issue is the voltage readings I am getting between the two O2 sensors.
The front is around 0.4 volts as it should be, but the post cat sensor is reading around 0.8v indicating a rich exhaust condition. Does this make sense? How could the exhaust go from lean to rich as it passes throught the cat? The pre-cat is reading lean, but the post cat is reading rich. Would this indicate a bad post cat sensor or is there something i am missing?
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Hammer Time
Ultimate Carjunky
/ Moderator
Jan 22, 2013, 10:05 AM
Post #4 of 8
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Re: 05 altima 2.5 incorrect oxygen sensor readings
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Either you are reading that before it heats up or whatever you are reading it with is not updating fast enough to catch the cross counting.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Discretesignals
Ultimate Carjunky
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Jan 22, 2013, 10:07 AM
Post #5 of 8
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Re: 05 altima 2.5 incorrect oxygen sensor readings
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The front sensor is zirconia, so it should be switching rapidly between 0-1 volts DC when it is fully warmed up. The rear sensor should stay steady during cruise after the catalyst has lit off.
What are you using to monitor O2 sensor voltage? If your using a scan tool, take into consideration that the frame rate of the tool is a lot slower than the actual O2 sensor voltage signal from the sensor itself.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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jrittenb
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Jan 22, 2013, 11:07 AM
Post #6 of 8
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Re: 05 altima 2.5 incorrect oxygen sensor readings
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I am using an android phone app called Torque and a usb adapter plugged into the OBD. I have checked the device against my other two cars and it seems to be operating correctly. Could it be that the post cat sensor is clogged?
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Hammer Time
Ultimate Carjunky
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Jan 22, 2013, 4:29 PM
Post #7 of 8
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Re: 05 altima 2.5 incorrect oxygen sensor readings
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That setup is useless for live data. It simply can't keep up with the data speed.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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nickwarner
Veteran
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Jan 24, 2013, 10:18 PM
Post #8 of 8
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Re: 05 altima 2.5 incorrect oxygen sensor readings
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To clarify HT, he and I both own a Snap-On Solus Pro scan tool. Its a top of the line professional tool with factory-level programming that costs $4000 new. It can display data in graphing mode as well as data pids, which is what you're doing with the app for your phone. But your phone got that waveform from the ecm, which had to process raw data from the O2 and then send it through a J1939 data bus to an OBDII port, from which it enters your phone and has to be processed further by your device to show on the screen. It isn't an oscilloscope reading of what is happening even though you can make it display like it is. Even the expensive scanners HT and I use still cannot come close to the refresh rate and data transfer of a scope. I've put my Solus up in graphing mode against a sensor and the scope could show a glitch in the signal when the scanner showed nothing.
Your post-cat O2 should read richer than the pre-cat if the cat is functional, and will not oscillate nearly like the pre-cat. This is because the cat uses oxygen in its chemical reactions with platinum, palladium and rhodium along with a lot of heat to convert CO, HC and NOx into water, CO2 and nitrogen. This should use up a lot of the oxygen passing the upstream sensor before reaching the downstream.
Your sensor operates on a 0-1v scale. 450mv is stoichometric, below it is lean and above it is rich. Try your app out to see it. Get on an empty street and go full throttle. You should see that pre cat sensor go above 800mv while you are at that throttle level as the ECM richens the mix and ignores O2 readings at WOT. But while sitting there running you can induce a massive vacuum leak like pulling the brake booster hose. You should hear it suddenly run like crap and the O2 voltage drop way low. In that test you have shown the O2 to be functioning properly and able to respond to its environment.
Your app can give you some helpful info but it is by no means a true scan tool or an oscilloscope and should not be viewed as such. I think it can be useful for some help in troubleshooting a system, but it isn't any automatic fix like some people think it is. No scanner on earth can tell you right away what is wrong with a car. If it did, we would all own one.
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