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Check engine light, code P0420 and hissing sound

Tiny
FITTYFIT
  • MEMBER
  • 2009 HONDA FIT
  • 4 CYL
  • 2WD
  • MANUAL
  • 160,000 MILES
Yesterday my check engine light went on, I immediately used a reader, and it is throwing the dreaded p0420. There are absolutely no performance issues whatsoever.

However, a couple days ago I decided to change my oil. I had just driven the car, so I had to wait for it to cool down a bit before I got under it. That is when I noticed a hissing sound under the car somewhere. I thought maybe it was the A/C and didn't think anything of it (not sure why because I've never noticed this before).

I also decided to clean the engine department under the hood on this day with some degreaser and water followed by a bit of tire foam. Not sure if this impacted anything.

What should I do?
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Tuesday, April 5th, 2022 AT 4:41 PM

8 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

If you have a code for a catalytic converter issue and also hear a hissing, it sounds like the converter is partially plugged. See if the sound is coming from the converter or the exhaust.

Also, take a look through this link and let me know if it mirrors anything you are experiencing:

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

Let me know.

Joe
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Tuesday, April 5th, 2022 AT 8:00 PM
Tiny
SCIONBOYZ
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  • 9 POSTS
I checked the link, but it didn't really have anything there that helped. Item number two on the link said check engine light but it didn't say anything other than what I already told you.

If it is partially blocked, what do I do? How can I test for that?
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Wednesday, April 6th, 2022 AT 1:18 AM
Tiny
SCIONBOYZ
  • MEMBER
  • 9 POSTS
Also, the hissing goes on a long time after I turn the engine off, at least 15 minutes you can still be heard when I pop the hood. You can't hear it without the hood pops and certainly not from inside the car though.
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Wednesday, April 6th, 2022 AT 1:20 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

That is odd. Do you hear it for 15 minutes only from under the hood? I wouldn't think that would be the exhaust. Is it loud enough for you to record and upload it for me to hear?

As far as the code is concerned, is the P0420 the only one stored? I ask because there are other things that can set it. See pic below.

If there aren't other codes, then it is likely the converter needs replaced.

Let me know if you can record it and if any of the codes listed below are stored.

Take care,

Joe
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Wednesday, April 6th, 2022 AT 4:23 PM
Tiny
FITTYFIT
  • MEMBER
  • 3 POSTS
Here I drove 1.5 miles and lifted the hood and heard nothing. Then I drove it back home with A/C on full blast and lifted the hood and the sound was back. Not sure if A/C had anything to do with it or it just needed to be driven more.
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Wednesday, April 6th, 2022 AT 11:05 PM
Tiny
FITTYFIT
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Also, I am only getting p0420 code. Today I purchased a bottle of duralube, erased the code from reader, put half of it in with 1/4 tank gas, drive around for like 75 miles until tank almost empty, went and put the rest of the bottle in and filled tank of gas, drive around a little more, code hasn't come back yet. No performance issues whatsoever.
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Wednesday, April 6th, 2022 AT 11:10 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

I believe what you are hearing is refrigerant gas in the expansion valve. I noticed you didn't hear it when the AC wasn't used, and you did when it was.

Unless you have a leak, the sound is not out of the ordinary. Please understand that it was hard to hear, but that is what I thought I heard. Pay attention to see if it only happens when the A/C is used. If that is the case, there shouldn't be anything to worry about.

Has the check engine light returned? If it hasn't, let's keep our fingers crossed. If it does return, let me know. If no other codes return at the same time, I suspect the converter will need to be replaced.

Let me know.

Take care,

Joe
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Thursday, April 7th, 2022 AT 1:01 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

Yes, I have heard of it and people always seemed to be happy with the results. Hopefully, this will help for a long time. I'm glad the light hasn't returned.

Let me know if that changes.

Take care of yourself and thanks for the update.

Joe
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Thursday, April 7th, 2022 AT 8:14 PM

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