Misfire codes P0420 and P0302?

Tiny
DQ671
  • MEMBER
  • 2007 TOYOTA COROLLA
  • 4 CYL
  • AUTOMATIC
  • 117,000 MILES
Been getting misfires recently. First started with a P0302. Then changed out all spark plugs and coil packs, then one day accelerated and went faster and then misfire came back but this time with P0420 catalyst. Again I swapped out all plugs again ran good but as I was going up hill I got a misfire again cylinder 2. So at this point just reaching out for some expert advice as to what might be causing these misfires or every time I go over 50 miles per hour it’ll misfire, and performance is sputtering. Thanks in advance, maybe y’all can point me in a direction to check next. Thank you
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Sunday, June 1st, 2025 AT 3:12 AM

3 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
When you pull the spark plug out of #2 cylinder does it look dark, or black compared to the others? If so, it sounds like the fuel injector needs to be replaced, here is how to remove the fuel rail to swap it out. Here is how to test the injector once it is removed:

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

Check out the images (below). Let us know how it goes.

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Sunday, June 1st, 2025 AT 10:58 AM
Tiny
DQ671
  • MEMBER
  • 20 POSTS
They all look pretty much the same greyish dark. But I’m going to check out the injectors next like your advice hopefully that solves it otherwise I’ll continue to learn as I go. Thank you I’ll update once I find the issue.
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Monday, June 2nd, 2025 AT 2:25 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Sounds good, I am here to help. Yep, the injector will be the next step. Does the engine run smoothly? If not, you might want to do a compression test on #2. here is a guide:

https://www.2carpros.com/articles/how-to-test-engine-compression

You can rent a gauge set at the auto parts store for free. Please go over this guide and get back to us.
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Monday, June 2nd, 2025 AT 11:35 AM

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