Exhaust 4 Cats required for smog in California?

C

Codosuarus

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Sep 28, 2025

#1

• Sep 28, 2025

• #1

I have a '91 GT that I upgraded quite a bit in about 2004 (before I had kids).
This included BBK shorty headers and an aftermarket exhaust system.
At the time, I did everything to make sure that I was california smog legal.
I have smogged the car every two years in california ever since.
I probably have put less than 10,000 miles on the car since.
I brought the car in for a smog the other day and the smog tech told me that 4 cats were required.
Now I remember this being mentioned back when I replaced the exhaust system and I dont remember any aftermarket exhaust systems having 4 cats.
But it has been so long that I cant remember what the solution was.
Can anybody remind me how these things were smogged with only two cats?

 

AeroCoupe

lube between the nut and the face. I know my lubes

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#2

• Sep 28, 2025

• #2

Car came with four cats from Ford so California will require four. Sounds like you got lucky prior to this.

Did the inspector ask for the CARB EO number for each piece of the exhaust system? If not then just weld two more in downstream of the O2 sensors and be done with it.

 

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Noobz347

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#3

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Now, I [think] there is one other system out there on the market with an H-pipe that is California legal. I could not find the one I was looking for but maybe somebody else in the forum knows which one I'm talking about.

I did find this one though:

Magnaflow Mustang Direct-Fit Catted X-Pipe; California Grade CARB Compliant 337338 (86-93 5.0L Mustang) - Free Shipping

FREE SHIPPING! CARB-Approved. When you need to change your catalytic converter and you drive in California, install the Magnaflow Direct-Fit California Catted X

www.americanmuscle.com

So, there are now two aftermarket, Cali compliant systems on the market that I'm aware of.

If I find the one with the H-pipe, I'll come back and post it.

 

Noobz347

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Eng H Pipe OE with 4 Catalytic Converter for Ford Mustang 1986-1993 5.0L | eBay

NOTE: Before buying any Converter, you must be aware that Converters are designed to last the life of a car or SUV / truck. If your converter needs to be replaced it is due to an emissions error/problem in your engine tuning.

www.ebay.com

 

AeroCoupe

lube between the nut and the face. I know my lubes

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#5

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I saw that one too but scroll to the bottom, they will not ship to California as the cats are not CARB. Its just like this one:

https://www.carid.com/sf-pl-mpn/pid-6473730396-mmyid-4377-ofmpnid-4478194182.html?gclsrc=aw.ds&gad_source=1&gad_campaignid=22264853177&gclid=CjwKCAjwuePGBhBZEiwAIGCVS3Epyhn7aaDqaOf7wc6OJlV11PMbjFl76-cKuscG3DulfXtnNBr6zBoCRU0QAvD_BwE

 

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Noobz347

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#6

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AeroCoupe said:

Scroll to the bottom, they will not ship to California as the cats are not CARB.

Click to expand...

Which ones?

 

AeroCoupe

lube between the nut and the face. I know my lubes

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#7

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• #7

The ones you posted.

 

gkomo

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#8

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I have the Magnaflow CARB legal xpipe. A very pretty penny, but it works.

 

Noobz347

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#9

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AeroCoupe said:

The ones you posted.

Click to expand...

I posted 2. You mean the Magnaflow from American Muscle?

I only see a Prop 65 warning.

 

Noobz347

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#10

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AeroCoupe said:

I saw that one too but scroll to the bottom, they will not ship to California as the cats are not CARB. Its just like this one:

https://www.carid.com/sf-pl-mpn/pid-6473730396-mmyid-4377-ofmpnid-4478194182.html?gclsrc=aw.ds&gad_source=1&gad_campaignid=22264853177&gclid=CjwKCAjwuePGBhBZEiwAIGCVS3Epyhn7aaDqaOf7wc6OJlV11PMbjFl76-cKuscG3DulfXtnNBr6zBoCRU0QAvD_BwE

Click to expand...

Ohhh... I see. nevermind that one then.


There is still an H-pipe version out there somewhere that is 50 state like the Magnaflow X. I recall it being pretty pricey too.

 

nickyb

I gotta say i never painted my nuts, Never Ever

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#11

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• #11

I'm one state over from cali. And no visual just a sniffer test in the tail pipes every year.
I've used the same guy for smog since I got my stang 18 years ago, I think he likes her that's why.

 

S

Scottyding73

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#12

• Oct 6, 2025

• #12

Yes unfortunately I believe magnaflow has these. I replaced mine 10 years ago and it was around $800 with a carb approved crossover pipe for even for California.