Mazda Millenia S 1999 - Service Codes P0421 and P0431?

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Bradley R

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We own a 1999 Mazda Millenia S. The check engine light comes on, we read the codes (P0421 and P0431) then reset it. Frequently we can drive the vehicle for hundreds of miles before the light comes on. Sometimes it comes on after less than 50 miles. My tester describes the codes as "Warm up catalyst below threshold". One code is for Bank 1, the other for Bank 2.

We've had quite a few problems with this vehicle including an engine rebuild at 85,000 miles. We've been to several mechanics including two Mazda dealerships. They all want to try replacing this and that. We've been down that road with several other repairs and I'm really looking for a solution, not a guessing game.

One dealer suggested that we need to replace the catalytic converters (both of them). Again, this is purely a guess.

The vehicle has 120,000 miles (40,000 on the new engine) and runs fine. I believe that if the catalytic converters were bad, the vehicle would run rough.

Could this be a problem with the electronics?
 
It's either your catalytic converter(s) or the secondary oxygen sensors.

Either the catalytic converters are not flowing efficiently enough or the secondary O2 sensors are too clogged/dirty to read correctly.

It's not uncommon to not feel any performance decrease from these codes because the the air/fuel ratio is determined by the primary O2 sensors. The only purpose of the secondary O2 sensor is to measure the efficiency of the catalyst. Purely emissions related...

So it's likely either one or the other...the catalyst of the O2 sensors...
 
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