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Hey guys im still sort of new to this forum and i need HELP!!

My check engine light came on the other day and today i checked with and OBD2 Scanner and it and gave me the DTC P0421 Warm Up CAT efficiency Low Bank1.

I want to know if anyone has had this DTC before and if so what did you do to fix it? Do I need to just replace the CAT? Any information regarding the P0421 Code would be very helpful to me as i'm new to the Miata game

About The Car
- 1999 Miata NB1 (All Stock)
- 5 Speed Trans.
- 108xxx miles
- And I recently replaced the upstream O2 Sensor
 

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At that age it probably needs a cat regardless. How old is the second O2? If you don't live in an emmisions state (ie: no yearly check for emmisions codes/compliance) just pull the CEL bulb out. No point wasting money on emissions, worst case you'll lose an mpg or 2 by running richer.
 

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I had the same code on my '99. I checked with my OBD2 scanner (which shows me the individual sensors) and saw the rear o2 sensor was dead. It could also mean a bad cat, or if the car feels like it's misfiring that code may apply for coil/plug/wire failure, but I don't recall off the top of my head.
 
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