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DTC P0421, P0431
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Gig2005
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PostPosted: Wed Jul 04, 2007 1:40 pm    Post subject: DTC P0421, P0431 Reply with quote

Hi guys,

Recently the CEL came on on my MPV 2002. Car reader shows P0421 and P0431, which is Warm-up catalyst system (RH) / (LH) efficiency below threshold. The van runs very well. Fuel consumption is about 13-14 L/100 km/city drive and below 10 L/100 km-hw drive. No bucking, no hezitation, no problems connected to engine performance. Last winter I had one of the flexes replaced. By some reason it burst open and obviously had to be replaced.

When the van is started, especially in winter, there is some kind of noise at the beginning. But then it goes away. I figured it might be the manifold. When the engine is cold the crack is wider, when the engine gets warmer, the crack closes. Maybe I'm mistaken. I was wondering how it can affect the cats and O2 sensors? Could it be the reason for the cat or O2 sensors to go bad? Is it absolutely necessary to do something about the manifold? How it could be better done with the dealer to have them diagnose the problem free of charge?

I seem to check all the posts related to the topic (P0321, P0431, the cat and O2sensor) on the forum but still feel a bit confused. Is it the Oxygen sensors that give me the trouble or the cats? Some people say it's the cat, the others say it's the O2 sensor. As the cat is covered by 8 yr /128,000 km warranty I'll be safe there.

I have an appointment with a Mazda dealer next week and they confirmed that the cats are covered and will be replaced at no cost to me, if they are faulty. I just wanted to get as much info as I could to be more knowledgable (and prepared to possible surprises on their part) on the issue. They will charge me around $100 for the CEL diagnostics. I told them that I know the DTC codes but it seems their policy to check the codes anyway and to get $100 for the service.

My concern is if it's Oxygen sensors that are faulty and not the cats I'd rather not go to the dealer and save $100, and replace the sensors myself. $100 does not seem to be a huge amount but I hate to pay it for nothing and if you think that this money can get you an O2 sensor, then you see things from a different perspective.

I will really appreciate everyone's input on this, especially those who had the same problem and the same DTC codes on.
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Gig2005
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PostPosted: Tue Jul 10, 2007 10:37 am    Post subject: Re: DTC P0421, P0431 Reply with quote

I guess noone had the same problem before?

So I was at a local Mazda dealer today. Their diagnostics confirmed that the cats were faulty and they would be covered by the warranty. I'll bring my MPV to them for repairs tomorrow and keep you updated on the progress.
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Gig2005
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PostPosted: Fri Jul 13, 2007 1:39 pm    Post subject: Re: DTC P0421, P0431 Reply with quote

So, I took my van to the dealer. They replaces two manifolds at no cost to me. On top, they did not charge me for the diagnostic test. It was covered by warranty as well, b/c it was related to the repairs under warranty. Go figure.

I'm a bit surprised though as to how come they replaced manifolds but not the cats? But I'm really happy that this issue is resolved. Does it mean that the manifolds were Ok all along? Or maybe the cats are attached to the manifolds? I don't know.
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bubba
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Joined: 06 Sep 2005
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Location: Southern, OR, USA

PostPosted: Fri Jul 13, 2007 6:45 pm    Post subject: Re: DTC P0421, P0431 Reply with quote

The front warm up cats that set the two codes are built right into both of the exhaust manifolds. You are very correct about the crack being wider when it is cold and then closes as it warms up. It was probably not cracked but manifold surface warps which is basically doing the same thing. Sounds like you got to kill two birds with one stone. No CEL and no more exhaust leak and noise on cold start.
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