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Alright so my mechanic thinks my code from the CEL is from a bad cat. (I’m not worried about the CEL so much as the car just running like crap.) his recommendation was to replace the cat and see if that fixes the situation. I’d like to get rid of the cat all together. Question is, if I delete the cat, will the car still run like a dud.
TIA!
 

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"his recommendation was to replace the cat and see if that fixes the situation."
Ask mechanic if he is willing to stand behind his diagnosis or is it just a lets see if it fixes it.
Car running like crap may be due to cylinder 4 misfire and not the P0420.
If single cylinder misfire then chances are the cat isn't causing a misfire only become less efficient and setting P0420.
Regarding the cat question, what does the state require?
 

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Regarding the cat question, what does the state require?
Amen to every blessed word you've said, but especially this proviso!!

Good heavens, why would anyone (and I'm blaming the mechanic here) propose replacing a catalytic converter if there is a code indicating a misfire on any given cylinder showing up? That's all it takes for a car to start running very noticeably "crappier" and the fewer cylinders you have to mask it the crappier losing one cylinder will be. And I can tell you, from previous experience with a Jaguar XJ8L that had ignition coils just die at random, that having one cylinder "out" will make the car run very, very badly indeed.
 

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Discussion Starter #6
"his recommendation was to replace the cat and see if that fixes the situation."
Ask mechanic if he is willing to stand behind his diagnosis or is it just a lets see if it fixes it.
Car running like crap may be due to cylinder 4 misfire and not the P0420.
If single cylinder misfire then chances are the cat isn't causing a misfire only become less efficient and setting P0420.
Regarding the cat question, what does the state require?
Yeah he told me that it’s possible that I’m getting the #4 cylinder misfire code from the cat being plugged or the substrate falling down into the pipe, but he wasn’t 100% sure. Also he noticed that the flex pipe was leaking so I took the car to the muffler shop to have that fixed first. The plugs and coil packs are brand new. Oh he also said he did a pressure test and that was good too.

2002 LE with 275k
 

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Discussion Starter #7
Yeah he told me that it’s possible that I’m getting the #4 cylinder misfire code from the cat being plugged or the substrate falling down into the pipe, but he wasn’t 100% sure. Also he noticed that the flex pipe was leaking so I took the car to the muffler shop to have that fixed first. The plugs and coil packs are brand new. Oh he also said he did a pressure test and that was good too.

2002 LE with 275k
Also, I live in a county where they don’t check emissions. As long as there’s no CEL, the wipers, lights & signals work then you’re good to go.
 

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A plugged cat generally provides slow vacuume on the take up when throttle is snapped. An inoperable cat has the same inlet and outlet temp(as opposed to being hotter on outlet when functioning).

A plugged cat would be very noticeable at full throttle conversely to idle, not that it wouldn't be a problem as well.

Hope that helps.

Cheers and stay safe from Canada!
 

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Plugged cats don’t usually cause cylinder misfires. How old are the plugs? How many miles on the car?
 

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@Eric Schuelzky are you doing any of this work yourself or are you just trying to validate what the mechanic is saying?

If you're doing some work I'd swap the #4 coil with a different one (say #1) and see if the code changes. You could also put the #4 plug in cylinder #2. Just because they are new doesn't mean you couldn't get a faulty part or have a premature failure. I assume when you say he did a "pressure test" that you are referring to a compression test to see how much pressure is developed in the cylinders? Or did he pressure test the coolant system to check for a possible head gasket leak. These will be distinctly different tests.
 

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+1 swap the coils and see if the misfire follows.
 

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Discussion Starter #14
@Eric Schuelzky are you doing any of this work yourself or are you just trying to validate what the mechanic is saying?

If you're doing some work I'd swap the #4 coil with a different one (say #1) and see if the code changes. You could also put the #4 plug in cylinder #2. Just because they are new doesn't mean you couldn't get a faulty part or have a premature failure. I assume when you say he did a "pressure test" that you are referring to a compression test to see how much pressure is developed in the cylinders? Or did he pressure test the coolant system to check for a possible head gasket leak. These will be distinctly different tests.
I ended up completely replacing the #3 & #4 plugs and coils since those were the codes that it was throwing. We’ll see how it does.
Also, about how long does it normally take for the idle to return to normal? It idles super low now. (Which to my understanding is normal after disconnecting the battery?)

also regarding the test...I don’t remember exactly. I’ll call him tomorrow and see. The cylinder test sounds like it would be the logical one.
 

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Discussion Starter #15
I ended up completely replacing the #3 & #4 plugs and coils since those were the codes that it was throwing. We’ll see how it does.
Also, about how long does it normally take for the idle to return to normal? It idles super low now. (Which to my understanding is normal after disconnecting the battery?)

also regarding the test...I don’t remember exactly. I’ll call him tomorrow and see. The cylinder test sounds like it would be the logical one.
After driving the car for about 25 miles the CEL is back with the cylinder 4 misfire p0304 and the p0420 codes.

Any other idea why I’d be getting the p0304 code along with p0420? I’d also still like to know if the car would run fine without a cat at all? Otherwise I guess I’ll be purchasing a new cat. Thoughts?
 

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You can get a misfire from spark, fuel, or air (or lack of those). Since coil/plug is new, you need to check the injector and compression. My car had a misfire on cylinder #2 and after checking the spark and injector, it turned out to be no compression due to a burned valve. Fix the misfire before investing in a new CAT (which you may need to replace anyway and possibly ruining it). Cheers!
 

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Fix the misfire before investing in a new CAT (which you may need to replace anyway and possibly ruining it).
Absolutely. I own a 1996 Buick Roadmaster that had a very similar issue for a while until we figured out that it was the actual plug wire, which wasn't obvious at first (or, really, even later). You can ruin your catalytic converter, but before you replace it, definitely replace both oxygen sensors.

I thought I had destroyed mine due to the amount of raw, unburned fuel that it had to deal with during the "misfire era." But it turns out I hadn't, but destroyed one of the two oxygen sensors, and I can't remember if it was the upstream or downstream, but I believe it was the latter. It's way cheaper to replace both to make sure that it isn't one of them before biting the bullet and getting a new catalytic converter.
 
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