Replacement Cat From Moss Miata - 1997 Mazda MX-5 Miata Long-Term Road Test

1997 Mazda MX-5 Miata Long-Term Road Test

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1997 Mazda MX-5 Miata: Replacement Cat From Moss Miata

May 3, 2013

1997 Mazda MX-5 Miata

If you, like us, reside in California, you'll quickly find that replacing your catalytic converter is a thorny issue. The replacement cat has to carry an exemption order from CARB to be allowed for sale in California (a so-called "50-state" cat), and not all catalytic converter manufacturers bother to certify their cats thusly. As a result, choices can sometimes be slim, especially if you drive a car that wasn't sold in great numbers in CA.

Hordes of Miatas were sold in CA, so for those cars it's less of a hassle. Still, to be smog-legal your new cat has to have a CARB EO number stamped in its heat shield, lest the smog technician will fail your car at its next smog check. You can go to, enter your car in their catalyst search tool and pore through the list of approved cats for a 50-state cat. Then search the resulting part numbers on the respective cat manufacturer's Web site to find that they don't make that part anymore or that it's been superseded.

Or you can make it nine thousand times easier on yourself and simply visit Moss Miata like we did. They'll sell you the right cat and send it to your door. It's dead simple.

1997 Mazda MX-5 Miata

Our old cat was done. Above are chunks of catalyst substrate that vomited out of it upon removal. This jacked-up mess was causing poor catalyst efficiency.

The bum cat was also causing a huge exhaust restriction, killing power and throttle response and making the car generally a bit unpleasant to drive. Up until the check engine light illuminated, I'd assumed our car was slowly losing compression.

1997 Mazda MX-5 Miata

Miata weight weenies, rejoice: the new cat from Moss Miata is physically smaller and lighter than the original one.

1997 Mazda MX-5 Miata

Your 50-state-legal cat must bear the CARB EO number the body to pass a smog check. That assumes the smog tech actually looks at your cat...

1997 Mazda MX-5 Miata

Of course, this happened. This stock exhaust nut galled and seized on the original cat's stud, even after a healthy drizzling of penetrant. Then, pa-tang! The stud snapped while hitting it with the impact wrench, and the now-liberated nut flew across the shop at Mach 6.

This, plus the too-short bolts found elsewhere on the original cat (thank you, previous owner) necessitated an impromptu walk to AutoZone for replacement hardware, in which you have to deal with their hilariously disorganized explosion of overpriced empty boxes where the metric hardware used to be.

Otherwise, this is an easy DIY install. The Moss Miata cat was a perfect fit. Four bolts and done.

Sources: Moss Miata

Jason Kavanagh, Engineering Editor


  • What was the driving difference? Pretty obvious?

  • I just replaced the cat on my '91 318is after it failed smog by a large margin. The replacement was CA compliant, though it was about an inch too long...I was kind of surprised the smog guy used a mirror and looked under the car at the cat. Didn't think they cared enough to check most of the time. My friend and I used a boroscope to inspect the old one. The honeycomb was intact, but it was chalky white like what you have in the picture. Are you guys going to recycle the cat?

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