# Bad O2 Sensor or Cat Problem? **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** hyundai, elantra\ **Created:** [July 26, 2020, 4:35pm UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895 "2020-07-26T16:35:50Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![ryan77627](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@ryan77627](https://community.cartalk.com/u/ryan77627)\ **Post date:** [July 28, 2020, 1:26pm UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/21 "2020-07-28T13:26:13Z")
Here is the graph from me idling after doing a lot of driving. This was the result after about 20 minutes of highway driving after 40 minutes of regular driving. It seems to be about the same. The downstream has too much activity with the upstream not having enough. I noticed that when I release the throttle the upstream sensor goes high and the downstream goes low for a bit before going back to regular ![Screenshot_20200727-215100_Car_Scanner](https://global.discourse-cdn.com/cartalk/original/3X/b/2/b22c271b34501bacd20a84c97f49382bf6bff480.png) ---
**Author:** ![db4690](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/db4690/32/2583_2.png) [@db4690](https://community.cartalk.com/u/db4690)\ **Post date:** [July 29, 2020, 12:46am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/22 "2020-07-29T00:46:50Z")
based on the incomplete data I’ve seen so far . . . you still haven’t shown fuel trims and loop status, for example . . . that upstream sensor is shot P0420 sets when the downstream sensor fluctuates at a rate nearly the same rate . . . around 80% . . . as the upstream sensor Whatever’s going on with the cat, I still think you need to replace the upstream sensor first and retest The cat may very well be degraded enough to cause that code to set, but I’d start with the small stuff first ---
**Author:** ![ryan77627](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@ryan77627](https://community.cartalk.com/u/ryan77627)\ **Post date:** [July 29, 2020, 1:42am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/23 "2020-07-29T01:42:57Z")
I don’t really have a way to monitor the loop status after I begin driving, all of this graphed data is captured while I’m driving and viewed later. I just know that the loop status goes to closed after around 15-25 seconds after turning the car on (a guess, don’t take those numbers as fact). Anything after I start driving is unknown to me. Attached is the same drive as last post, with the O2S1 Short term Fuel trim data plotted as well. I will post LTFT tomorrow when I go out again. I am planning on buying new O2 sensors, both if they aren’t too expensive and will replace them both when they arrive, since if one went out the other probably isn’t the farthest behind. I will re-evaluate afterwards. I don’t need the catalyst to be the most efficient in the world, just efficient enough to get inspected for the year. My hope is that a new upstream O2 sensor will work correctly and begin fluctuating just enough so that the downstream sensor fluctuating isn’t enough to set off the code. ![Screenshot_20200728-213756_Car_Scanner](https://global.discourse-cdn.com/cartalk/original/3X/6/c/6c5066b6a261be0ab1c3d4249ffd6cf885db4f9c.png) ---
**Author:** ![Nevada\_545](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/nevada_545/32/41232_2.png) [@Nevada\_545](https://community.cartalk.com/u/Nevada_545)\ **Post date:** [July 29, 2020, 2:14am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/24 "2020-07-29T02:14:54Z")
The time line in those recordings is rather compressed, you have 2 minutes of data packed into each recording, that makes it difficult to judge the reaction of the sensors. Reduce the increments from 20 seconds to 5 seconds, record the data at a steady 2000 RPMs, don’t snap the throttle, we can’t see the TPS value to know why there are big spikes in the graph. ---
**Author:** ![ryan77627](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@ryan77627](https://community.cartalk.com/u/ryan77627)\ **Post date:** [July 29, 2020, 3:42am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/25 "2020-07-29T03:42:42Z")
Okay, just so I’m sure I get everything tomorrow. You guys want a graph from the car after some heavy driving so everything is warm, with the engine at a steady 2000RPM, showing the TPS, short term fuel trim, and both O2 sensors, with as much data was I can show with 5 second intervals? Along with that I should state the Long term fuel trim and the loop status at the time of the recording? ---
**Author:** ![Nevada\_545](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/nevada_545/32/41232_2.png) [@Nevada\_545](https://community.cartalk.com/u/Nevada_545)\ **Post date:** [July 29, 2020, 3:51am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/26 "2020-07-29T03:51:23Z")
5 minutes of normal driving is sufficient. If you don’t snap the throttle open/closed there is no need for TPS display, snapping the throttle produces an erratic display, it shows the range of the oxygen sensors but makes for a confusing display. ---
**Author:** ![db4690](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/db4690/32/2583_2.png) [@db4690](https://community.cartalk.com/u/db4690)\ **Post date:** [July 29, 2020, 4:00am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/27 "2020-07-29T04:00:35Z")
I only mentioned the snap throttle test as a possible means of gauging the upstream oxygen sensor’s present condition And from what I’m seeing . . . so far . . . I’m really beginning to think the upstream sensor is done for ---
**Author:** ![ryan77627](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@ryan77627](https://community.cartalk.com/u/ryan77627)\ **Post date:** [July 30, 2020, 3:16am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/28 "2020-07-30T03:16:13Z")
Hello everyone. Here’s the data I would get today. Everything was taken with the engine hot directly after driving and at a (mostly) steady 2500RPM. Top graph in this picture is the STFT: ![Screenshot_20200729-231122_Car_Scanner](https://global.discourse-cdn.com/cartalk/original/3X/6/8/6853ee730166fc25dd003d678b2f6d574f197a2c.png) ---
**Author:** ![ryan77627](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@ryan77627](https://community.cartalk.com/u/ryan77627)\ **Post date:** [July 30, 2020, 3:16am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/29 "2020-07-30T03:16:44Z")
Here’s the next image (I can only post one image at a time still) ![Screenshot_20200729-231029_Car_Scanner](https://global.discourse-cdn.com/cartalk/original/3X/f/e/fe3b0366f3c1b27c53d951250b7417034dd4aef1.png) ---
**Author:** ![ryan77627](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@ryan77627](https://community.cartalk.com/u/ryan77627)\ **Post date:** [July 30, 2020, 3:17am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/30 "2020-07-30T03:17:21Z")
Here’s the last showing the LTFT and the loop status ![Screenshot_20200729-130855_Car_Scanner](https://global.discourse-cdn.com/cartalk/original/3X/0/2/02cfd60bc5622a604b9854233f3f0bbf4d4f23bb.png) ---
**Author:** ![ryan77627](https://avatars.discourse-cdn.com/v4/letter/r/b5a626/32.png) [@ryan77627](https://community.cartalk.com/u/ryan77627)\ **Post date:** [August 5, 2020, 1:41am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/31 "2020-08-05T01:41:34Z")
So I got the new O2 sensors and installed them today. I did some driving and I still got a pending P0420 code. The upstream graph had more activity and more definitive ups and downs, but it was still within that 0.48 to 0.53V range. Could it be a wiring problem? I’m beginning to stress though, because I’m going to college in about a week and would really like to have this problem situated and don’t have the real means to replace the cat at the moment… Anyone know of ways to get the cat to be just a little more efficient temporarily, just so I can get my car inspected for the year, I just need more time to get everything situated before I can begin putting money like that into my car. ---
**Author:** ![kurtwm1](https://avatars.discourse-cdn.com/v4/letter/k/13edae/32.png) [@kurtwm1](https://community.cartalk.com/u/kurtwm1)\ **Post date:** [August 6, 2020, 1:42am UTC](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895/32 "2020-08-06T01:42:57Z")
you could try a product named Cataclean or something like this: > **[Catalytic converter cleaner pass emissions Motor Power care high quality |...](https://www.ebay.com/itm/Catalytic-converter-cleaner-pass-emissions-Motor-Power-care-high-quality/323496885593?_trkparms=aid=555018&algo=PL.SIM&ao=2&asc=20160908110712&meid=2cdf3510cc8744c3a54ae94440c5b5a5&pid=100677&rk=2&rkt=30&mehot=pp&sd=323498469218&itm=323496885593&pmt=1&noa=0&pg=2386202&algv=SimplAMLv5PairwiseWeb&_trksid=p2386202.c100677.m4598)** > > With regular use, it provides protection against renewed contamination, increases fuel efficiency, optimizes the engine performance and ensures the proper functioning of the catalyst and the lambda sensor. [Previous page](https://community.cartalk.com/t/bad-o2-sensor-or-cat-problem/167895.md?page=1)