# Check engine light comes on after I fill up my gas tank **URL:** \ **Category:** Maintenance/Repairs\ **Created:** [December 11, 2019, 4:13pm UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471 "2019-12-11T16:13:31Z")\ **Posts on this page:** 11\ **Page:** 2
**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:** [December 22, 2019, 4:54am UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/21 "2019-12-22T04:54:35Z")
You’re taking me out of context here. ---
**Author:** ![ndswizz84\_160797](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/ndswizz84_160797/32/18614_2.png) [@ndswizz84\_160797](https://community.cartalk.com/u/ndswizz84_160797)\ **Post date:** [December 22, 2019, 4:58am UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/22 "2019-12-22T04:58:37Z")
Thanks! I was trying to avoid replacing the catalytic converter. Perhaps I’ll get lucky and find out that’s not the issue. _fingers crossed_ ---
**Author:** ![ndswizz84\_160797](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/ndswizz84_160797/32/18614_2.png) [@ndswizz84\_160797](https://community.cartalk.com/u/ndswizz84_160797)\ **Post date:** [December 22, 2019, 5:01am UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/23 "2019-12-22T05:01:50Z")
It’s a 4 cylinder. And the code has always been the P0420. the mechanic told me that it reported that only the top O2 sensor needed to be replaced, but he felt I needed to change both. ---
**Author:** ![ndswizz84\_160797](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/ndswizz84_160797/32/18614_2.png) [@ndswizz84\_160797](https://community.cartalk.com/u/ndswizz84_160797)\ **Post date:** [December 22, 2019, 5:03am UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/24 "2019-12-22T05:03:54Z")
Thanks, everyone, for your help. I’ll keep you updated. For now, I’ll just drive my dad’s Explorer out of town and deal with this when I return. Happy holidays!! 😄 ---
**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:** [December 22, 2019, 5:15am UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/25 "2019-12-22T05:15:46Z")
If you take this vehicle on your trip the check engine light will likely turn off, catalyst efficiency generally improves with highway/higher temperature operation. I drove my car for a year and a half with an intermittent P0420 fault code, it is not something that would bother me. ---
**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:** [December 22, 2019, 3:40pm UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/26 "2019-12-22T15:40:12Z")
> [@ndswizz84\_160797](#): > > For now, I’ll just drive my dad’s Explorer out of town and deal with this when I return. By the way . . . do you even live in an area with smog inspections? If you don’t, I see no reason to deal with this right now . . . maybe not ever ---
**Author:** ![VDCdriver](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/vdcdriver/32/48514_2.png) [@VDCdriver](https://community.cartalk.com/u/VDCdriver)\ **Post date:** [December 22, 2019, 4:13pm UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/27 "2019-12-22T16:13:05Z")
> [@eddo](#): > > the fine folks at Autozone are not mechanics. +1 Accepting automotive advice from the guys at Autozone would be tantamount to taking healthcare advice from the cashier at CVS or Walgreens. ---
**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:** [December 23, 2019, 11:08pm UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/28 "2019-12-23T23:08:01Z")
> [@ndswizz84\_160797](#): > > the mechanic told me that it reported that only the top O2 sensor needed to be replaced, I’m curious where he got that. Usually it’s the the downstream sensor (bank 1, sensor 2) that causes the problem but if you replaced both, that is covered. P0420 does not specify any part, it just means that the cat efficiency is below threshold. What it is looking at is that it expects the output from the first sensor (bank 1 sensor 1) will swing through its voltage range about once a second. The downstream sensor (B1S2) will swing through its voltage range about once every 8 seconds or so. ---
**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 23, 2019, 11:28pm UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/29 "2019-12-23T23:28:09Z")
> [@keith](#): > > The downstream sensor (B1S2) will swing through its voltage range about once every 8 seconds or so. ![](https://global.discourse-cdn.com/cartalk/original/2X/5/56d6a6ecf077268887ecebf6dc15cfd108ce0bda.png) Tester ---
**Author:** ![ndswizz84\_160797](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/ndswizz84_160797/32/18614_2.png) [@ndswizz84\_160797](https://community.cartalk.com/u/ndswizz84_160797)\ **Post date:** [January 1, 2020, 6:00pm UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/30 "2020-01-01T18:00:35Z")
Happy new year! I want to thank you all for your input and suggestions. As a follow-up, I ended up disconnecting the battery (as someone suggested), risked it all and drove to Jacksonville, FL. AND THE VERDICT IS … no problems!! The engine light did not come on and my gas mileage was excellent! I am now at 120k and all seems well. Thanks, again!! ---
**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:** [January 1, 2020, 6:50pm UTC](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471/31 "2020-01-01T18:50:06Z")
Congratulations and thank you for the feedback. The trip probably did as much good for the vehicle as anything else did though. The constant stream of hot exhaust gasses going through the converter may have removed some of the deposits that have been building up in it. [Previous page](https://community.cartalk.com/t/check-engine-light-comes-on-after-i-fill-up-my-gas-tank/154471.md?page=1)