# Oxygen Sensor Graphs - Please help

**URL:** <https://community.cartalk.com/t/oxygen-sensor-graphs-please-help/177397>\
**Category:** Maintenance/Repairs\
**Tags:** nissan, altima, check-engine-light\
**Created:** [June 11, 2021, 3:47pm UTC](https://community.cartalk.com/t/oxygen-sensor-graphs-please-help/177397 "2021-06-11T15:47:25Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Shashi](https://avatars.discourse-cdn.com/v4/letter/s/ecb155/32.png) [@Shashi](https://community.cartalk.com/u/Shashi)\
**Post date:** [June 11, 2021, 3:47pm UTC](https://community.cartalk.com/t/oxygen-sensor-graphs-please-help/177397/1 "2021-06-11T15:47:25Z")

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Can anyone please help me understand my Pre-Cat and Post-Cat Oxygen sensor outputs! I have a P0420 error code. I have done everything (NEW spark plugs, ignition coil, Cataclean, servicing etc.) except put in a new-cat…but still cannot get rid of the code. Please help me diagnose the Oxygen sensor codes, so I can make a decision on what next! (see attached: Idle and ~2000 rpm)

 ![Screenshot_Idle](https://global.discourse-cdn.com/cartalk/original/3X/5/1/5192e9ec3a5b3031025eabbfe8816f6c9e331a54.jpeg)  
 ![Screenshot_2000 RPM](https://global.discourse-cdn.com/cartalk/original/3X/c/b/cbc880e08ae546a9f7004bbdd6ba22a57e4779f7.jpeg)

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**Author:** ![Tester](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/tester/32/15976_2.png) [@Tester](https://community.cartalk.com/u/Tester)\
**Post date:** [June 11, 2021, 3:52pm UTC](https://community.cartalk.com/t/oxygen-sensor-graphs-please-help/177397/2 "2021-06-11T15:52:41Z")

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![image](https://global.discourse-cdn.com/cartalk/original/3X/4/2/427a1c6c16ffa20a687d7be4513eec405c616bfd.jpeg)

Tester

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<div class="post-metadata">

**Author:** ![Shashi](https://avatars.discourse-cdn.com/v4/letter/s/ecb155/32.png) [@Shashi](https://community.cartalk.com/u/Shashi)\
**Post date:** [June 11, 2021, 3:59pm UTC](https://community.cartalk.com/t/oxygen-sensor-graphs-please-help/177397/3 "2021-06-11T15:59:48Z")

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Hello, Thanks for your reply. I have seen these a lot on the internet. Can you please look at my O2 readouts and help me understand if it is good or bad? My post-cat seems to run low voltage when idling…is that a good thing?

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**Author:** ![Tester](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/tester/32/15976_2.png) [@Tester](https://community.cartalk.com/u/Tester)\
**Post date:** [June 11, 2021, 4:55pm UTC](https://community.cartalk.com/t/oxygen-sensor-graphs-please-help/177397/4 "2021-06-11T16:55:12Z")

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The downstream O2 sensor is indicating the cat bad.

Tester

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**Author:** ![keith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/keith/32/5518_2.png) [@keith](https://community.cartalk.com/u/keith)\
**Post date:** [June 12, 2021, 10:59pm UTC](https://community.cartalk.com/t/oxygen-sensor-graphs-please-help/177397/5 "2021-06-12T22:59:52Z")

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I’d replace the downstream sensor first. It is responsible for about 60% of P0420 codes.

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**Author:** ![Shashi](https://avatars.discourse-cdn.com/v4/letter/s/ecb155/32.png) [@Shashi](https://community.cartalk.com/u/Shashi)\
**Post date:** [June 14, 2021, 4:35pm UTC](https://community.cartalk.com/t/oxygen-sensor-graphs-please-help/177397/6 "2021-06-14T16:35:00Z")

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That makes so much sense (logically). I changed both the sensors about 4 years ago (1 year apart from each other)…that is a cheap and easy fix/gamble before I do anything else. Thanks 🙂