# Check engine code P420 **URL:** \ **Category:** Maintenance/Repairs\ **Created:** [December 18, 2022, 1:18am UTC](https://community.cartalk.com/t/check-engine-code-p420/187985 "2022-12-18T01:18:14Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![peacew6\_183373](https://avatars.discourse-cdn.com/v4/letter/p/9dc877/32.png) [@peacew6\_183373](https://community.cartalk.com/u/peacew6_183373)\ **Post date:** [December 18, 2022, 1:18am UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/1 "2022-12-18T01:18:14Z")
HI, I’ve been getting P420 on my Hyundai Elantra 2004 (about 130K miles). I got scanned data below. Would you tell me if any number number is way off the normal range? FUEL SYS1 CL LOAD\_PCT 13.7 ECT – SHRTFT 0.8 LONGFT 6.3 RPM 767 SPARKADV 4.5 IAT(C) 9 MAF(g/s) 2.29 TP(%) 6.7 O2SLOC B1S12–B2S O2B1S1 0.490 SHRTFTB1S(%) 3.1 O2B1S2(v) 0.035 SHRTFTB2S(%) N/A OBSDUP OBDII _G=GREEN CHECKMARK S=STOP SIGN X=X MARK MIS G FUE G COM G CAT G EVAP X O2S X HRT G_ ---
**Author:** ![weekend-warrior](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/weekend-warrior/32/26122_2.png) [@weekend-warrior](https://community.cartalk.com/u/weekend-warrior)\ **Post date:** [December 18, 2022, 1:35am UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/2 "2022-12-18T01:35:42Z")
> [@peacew6\_183373](#): > > SHRTFTB2S(%) N/A I believe this is the short-term fuel trim set based on the downstream O2 sensor. n/a could mean there is a fault in sensor or wiring. but others here will know better ---
**Author:** ![PvtPublic](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/pvtpublic/32/13089_2.png) [@PvtPublic](https://community.cartalk.com/u/PvtPublic)\ **Post date:** [December 18, 2022, 11:53am UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/3 "2022-12-18T11:53:38Z")
If it’s a 4cyl. There is no Bank 2 sensor. So I guess N/A could be expected. If people can’t say the engine in their car, even if there was only one option, I ain’t wasting my time finding out. ---
**Author:** ![jtsanders](https://avatars.discourse-cdn.com/v4/letter/j/c2a13f/32.png) [@jtsanders](https://community.cartalk.com/u/jtsanders)\ **Post date:** [December 18, 2022, 1:27pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/4 "2022-12-18T13:27:57Z")
The only engine available in the US was the 2.0L I-4. ---
**Author:** ![VOLVO-V70](https://avatars.discourse-cdn.com/v4/letter/v/8e8cbc/32.png) [@VOLVO-V70](https://community.cartalk.com/u/VOLVO-V70)\ **Post date:** [December 18, 2022, 1:31pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/5 "2022-12-18T13:31:39Z")
The OP might not be in the US . ---
**Author:** ![jtsanders](https://avatars.discourse-cdn.com/v4/letter/j/c2a13f/32.png) [@jtsanders](https://community.cartalk.com/u/jtsanders)\ **Post date:** [December 18, 2022, 1:49pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/6 "2022-12-18T13:49:40Z")
All engines were I-4 regardless of where sold. ---
**Author:** ![VOLVO-V70](https://avatars.discourse-cdn.com/v4/letter/v/8e8cbc/32.png) [@VOLVO-V70](https://community.cartalk.com/u/VOLVO-V70)\ **Post date:** [December 18, 2022, 1:53pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/7 "2022-12-18T13:53:04Z")
Edmunds lists the Sonata of some years having a V6 . ---
**Author:** ![jtsanders](https://avatars.discourse-cdn.com/v4/letter/j/c2a13f/32.png) [@jtsanders](https://community.cartalk.com/u/jtsanders)\ **Post date:** [December 18, 2022, 2:15pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/8 "2022-12-18T14:15:35Z")
The wiki shows 4 I-4s in 2004 worldwide. ---
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [December 18, 2022, 10:20pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/9 "2022-12-18T22:20:51Z")
> [@peacew6\_183373](#): > > f any number number is way off the normal range? I don’t see anything way off. But the 5th line, long term fuel trim of 6.3 %, that is on the high side, probably pointing to some problem brewing at least. It means the drivetrain computer is being forced to inject 6.3% more fuel to meet the O2 sensor fuel mixture requirement, than it would otherwise based only on the intake airflow. Pretty good chance air is leaking into the engine somewhere, bypassing the airflow sensor. Check the intake system air boot, see any splits? Ask your shop to check the pcv system, problems could allow an air leak. Exhaust leaks could cause it too. p0420 means the signals from the pre- and post-cat o2 sensors are not matching like they would if the cat was working properly. The best way to diagnose that is for a shop to monitor those two signals side by side using a scan tool. Could be a faulty cat, or could be whatever’s causing the higher than expected fuel trim. O2 sensor failure possible, but unlikely. My guess, the extra gas has been injected over a long enough time that an incorrect fuel mixture has damaged the cat, and you’ll have to fix what’s causing the fuel trim problem along with replacing the cat. ---
**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:** [December 18, 2022, 11:03pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/10 "2022-12-18T23:03:37Z")
> [@George\_San\_Jose1](#): > > p0420 means the signals from the pre- and post-cat o2 sensors are not matching like they would if the cat was working properly. Have someone connect a scanner that can display real-time data and monitor the up-stream and down-stream O2 sensors, If the down stream O2 sensor matches the up-stream O2 sensor, the vehicle needs a new catalytic converter. [![](https://global.discourse-cdn.com/cartalk/original/3X/9/2/920c82aae79279b7169d496b337e65aa8eb81476.jpeg "Using upstream and downstream oxygen sensors to test the catalytic converter") ](https://www.youtube.com/watch?v=KGpMzdEO5KY) Tester ---
**Author:** ![peacew6\_183373](https://avatars.discourse-cdn.com/v4/letter/p/9dc877/32.png) [@peacew6\_183373](https://community.cartalk.com/u/peacew6_183373)\ **Post date:** [December 19, 2022, 4:31pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/11 "2022-12-19T16:31:13Z")
Thank you, George\_San\_Jose and Tester, for the reply. I checked the intake system air boot. Pretty good. No crack or split. All clamped down tight. I’m going to check the O2 sensor before I check other things since o2 sensor seems to be the easiest and cheapest part to replace. One thing though, you said p420 means the pre and post o2 are NOT matching. In a normal situation, if I didn’t have the P420 code and If my CAT and the O2 sensors were all good, shouldn’t the pre and post o2 sensor volt readings be **different**? As “Tester” has said if the pre and post O2 readings were matching, that might mean the cat is not doing its job. Am I correct? ---
**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:** [December 19, 2022, 4:39pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/12 "2022-12-19T16:39:15Z")
So, you’re going to replace O2 sensors, even tho there’s no O2 sensor codes? 🙄 Tester ---
**Author:** ![peacew6\_183373](https://avatars.discourse-cdn.com/v4/letter/p/9dc877/32.png) [@peacew6\_183373](https://community.cartalk.com/u/peacew6_183373)\ **Post date:** [December 19, 2022, 5:20pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/13 "2022-12-19T17:20:17Z")
Since George\_San said O2 sensor failure is possible and I got the red x mark for the O2S, I thought I give that a try. Tester, you suggested graphing the live data of pre and post O2 voltages to see if they match. As I posted originally, O2B1S1 = 0.490 & O2B1S2(v) = 0.035. Doesn’t that mean they do not match? ---
**Author:** ![VOLVO-V70](https://avatars.discourse-cdn.com/v4/letter/v/8e8cbc/32.png) [@VOLVO-V70](https://community.cartalk.com/u/VOLVO-V70)\ **Post date:** [December 19, 2022, 6:02pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/14 "2022-12-19T18:02:26Z")
> [@peacew6\_183373](#): > > Since George\_San said O2 sensor failure is possible I am going to get flagged but I don’t care. George is the resident guesser for this Forum . ---
**Author:** ![weekend-warrior](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/weekend-warrior/32/26122_2.png) [@weekend-warrior](https://community.cartalk.com/u/weekend-warrior)\ **Post date:** [December 19, 2022, 6:06pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/15 "2022-12-19T18:06:18Z")
In his defense, a lot of mechanical work is guessing and figuring the problem out. isn’t it? especially on a forum that you can not see the vehicle. ---
**Author:** ![TheWonderful90s](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/thewonderful90s/32/29584_2.png) [@TheWonderful90s](https://community.cartalk.com/u/TheWonderful90s)\ **Post date:** [December 19, 2022, 7:10pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/16 "2022-12-19T19:10:47Z")
I was expecting some P420 Marijuanna joke! ---
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [December 19, 2022, 9:21pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/17 "2022-12-19T21:21:13Z")
> [@peacew6\_183373](#): > > If my CAT and the O2 sensors were all good, shouldn’t the pre and post o2 sensor volt readings be **different**? Yes, but the o2 sensor’s entire signal is used for the comparison (voltage vs time). The voltages might be the same at certain instants of time. But the cat’s functionality can’t be determined by one voltage at one time. You need a way to chart the time history of both signals. By “match” I didn’t mean the voltages have to be exactly the same at all instances of time. I meant the relationship between the two voltage-vs-time signals has to match the specification. ---
**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:** [December 19, 2022, 9:33pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/18 "2022-12-19T21:33:27Z")
> [@George\_San\_Jose1](#): > > p0420 means the signals from the pre- and post-cat o2 sensors are not matching like they would if the cat was working properly. No George. That’s what you typed. And that’s misinformation! **PERIOD!** Tester ---
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [December 19, 2022, 9:37pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/19 "2022-12-19T21:37:47Z")
What do you other folks think? Did I provide complete misinformation? Or did I provide something mostly useful to the OP? ---
**Author:** ![VOLVO-V70](https://avatars.discourse-cdn.com/v4/letter/v/8e8cbc/32.png) [@VOLVO-V70](https://community.cartalk.com/u/VOLVO-V70)\ **Post date:** [December 19, 2022, 10:31pm UTC](https://community.cartalk.com/t/check-engine-code-p420/187985/20 "2022-12-19T22:31:02Z")
> [@George\_San\_Jose1](#): > > Did I provide complete misinformation? George try something new . If you are not sure of what you are posting then just do not post . You don’t have to post to every question that comes here . [Next page](https://community.cartalk.com/t/check-engine-code-p420/187985.md?page=2)