2014~2018: Code P0421...Is this the part I need?

[ZDA3]

I had recently replaced the catalytic converter of my 2015 mazda 3, plus also both the O2 sensors. It has 200,000 miles. I didn't know it also has a second converter under the hood with a manifold. I believe prior to the P0421 code occurring, it was a P0420 code first which was why I decided to have the catalytic converter replaced to begin with.

Is this the part I need:

2014-2016 Mazda 3 Exhaust Manifold PEB5-20-50X | Mazda Parts HQ

2014-2016 Mazda 3 Exhaust Manifold PEB5-20-50X | Mazda Parts HQ

2014-2016 Mazda 3 Exhaust Manifold PEB5-20-50X

www.mazdapartshq.com

Or this:

Sierra MANIFOLD 18-1957-2: Advance Auto Parts

Advance Auto Parts - Down for Maintenance

 

[negusm]

P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1).
P0421 stands for “Warm Up Catalyst Efficiency Below Threshold (Bank 1).

The Advance Auto one doesn't look like it has the cat. I for one, for things like this, call the local dealer's parts department with my VIN and have them look it up. You don't have to buy it from them but they are always happy to look up the part number. I run across odd mid year manufacturing changes all the time. You just can't know unless the dealer runs your VIN.

So those are the only 2 codes? No misfire or any other codes? They do point towards a bad CAT but you have to be careful. Did you call the dealer and verify the part number for those sensors? Did you use MAZDA sensors? Are you sure they were both O2 sensors? Some cars are O2 and some are Air/Fuel sensors. They look exactly the same. Again, the dealer will know.

The way I understand it, is that the ECM compares output from the first O2 sensor to the second O2 sensor. Replacing the CAT in between the two sensors, including the O2 sensors should have fixed the issue.

 

[ZDA3]

It did fix the P0420 issue but the P0421 code popped up out of nowhere. I'm told that the manifold converter should also be replaced as well.

The catalytic converter that was replaced was after market by Bosal. I'm not sure why the box has the Eastern Catalytic brand imprinted on its box though perhaps they manufacture Bosal's catalytic converters.

P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1).
P0421 stands for “Warm Up Catalyst Efficiency Below Threshold (Bank 1).

The Advance Auto one doesn't look like it has the cat. I for one, for things like this, call the local dealer's parts department with my VIN and have them look it up. You don't have to buy it from them but they are always happy to look up the part number. I run across odd mid year manufacturing changes all the time. You just can't know unless the dealer runs your VIN.

So those are the only 2 codes? No misfire or any other codes? They do point towards a bad CAT but you have to be careful. Did you call the dealer and verify the part number for those sensors? Did you use MAZDA sensors? Are you sure they were both O2 sensors? Some cars are O2 and some are Air/Fuel sensors. They look exactly the same. Again, the dealer will know.

The way I understand it, is that the ECM compares output from the first O2 sensor to the second O2 sensor. Replacing the CAT in between the two sensors, including the O2 sensors should have fixed the issue.

 

[ZDA3]

Anyway, the check engine light would disappear in and out but so far it's gone. I'm contemplating if I should even bother replacing the manifold converter...

I'm trading in the car in 1 or 2 years from now. It's a 2015 Mazda 3 isport, 2.0 with about 221,000 miles.