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Got a 2006 Mazda 6 3.0, code "P0431"

Was wondering if a bad after market upstreams sensor would make this code appear? The cat it's saying has that code is the front pre-cat. I've looked down the top hole for the upstream sensor, doesn't look clogged and i'm not having that many performance issues or anything else. Either than a rough cold start until I give her some gas and she warms up a little. Any help? Thanks!
 

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Forgot to say when I put it to my scanner and was watching the o2 sensor isolate up and down, it was a random reading it wasnt going up and down but the downstream was reading correctly.
 

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It's possible. When it comes to sensors, always go for OEM manufacturer because the ECU is sensitive.

Don't just do visual inspection on the cats.

Do a backpressure test on all four catalytic converters. Should be less than 1psi for unlogged cats.


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Got a 2006 Mazda 6 3.0, code "P0431"

Was wondering if a bad after market upstreams sensor would make this code appear? The cat it's saying has that code is the front pre-cat. I've looked down the top hole for the upstream sensor, doesn't look clogged and i'm not having that many performance issues or anything else. Either than a rough cold start until I give her some gas and she warms up a little. Any help? Thanks!
Technically, it could cause the code. The computer monitors the number of inversion that the upstream sensor makes in voltage and compares it to the downstream sensor. If the inversions are within a set ratio, and too close, it sends the code. Generally speaking though, No, a bad upstream sensor will not give you a P0431 (or P0421) code unless it is reporting its own failure. Since you will need to replace all the sensors anyhow, if you plan on repairing the car, it may be a good idea to start with those before biting off the big purchase.

As Os sensors age they slow in their response times. The lifespan of the 4-wire NTK sensors that the Mazda comes with is 100,000mi.
The upstream sensor (sensor 1) should be oscillating regularly between 0.1V and 1.0V. This sensor is controlling the fuel levels the engine is receiving. A fest moving sensor (newer) will increase accuracy of your fuel delivery.
The downstream sensor (sensor 2) should be very stable. A stable and relatively flat line, (say around 0.6V) would indicate that your catalytic converter is working correctly and passing a uniformly reactive exhaust.
When the downstream sensor is mimicking the upstream sensor by oscillating in sync (albeit with a slight delay in timing) that means the converter is no longer working correctly and needs to be replaced.
The fact that the matrix looks okay when you look at it is good news for your motor as it means that it has not started to disintegrate yet.
Catalytic converters don't fail, they are murdered. It is imperative that you fix the cause of too much fuel being sent into the exhaust to begin with (bad ignition coils, plugs, vacuum leaks, clogged injector) or it will destroy your replacement emission equipment as well.

If you are planning on keeping the car, and need to have operational emissions equipment, plan on replacing both manifolds, both downstream cats, all 4 oxygen sensors and likely coils and plugs (if not already replaced). You will want to check the PCV, air cleaner and intake accordion tube as well.

This is a common dilemma with the V6 and should probably have new coils and plugs as periodic maintenance items every 60,000mi (IMHO).

I have walked this journey twice and you can read about my mistakes here and here.

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