Code 431 on Millenia 2.5

LAKEWOOD

MEMBER
2001 MAZDA MILLENIA
6 CYL
2WD
AUTOMATIC
130,000 MILES

I recently replaced my 135,000 mile Millenia engine with one with 65,000 due to blown headgasket. Runs great but I am getting a code PO431. Code states warm-up catalyst effeciency below threshold (Bank 2). Is bank 2 the one near the radiator (front) or the firewall (back)? It appears this could be either the cat or a sensor - if so would it be the pre-cat or main cat and if the sensor would it be the sensor in the manifold before the pre-cat or the one after the pre-cat (prior to the main cat)? The local dealer is running a $1 special on code reads - thought I would take him up on it to see what I can learn from them as well. My plan now is to replace a sensor first (about $60) and then the pre-cat or main depending on which is most likely to be bad/causing my code. PS: Checked ex back-pressure last week - seemed pretty low.

3 Replies

HMAC300

MECHANIC
48,601 POSTS

That means that you converter lost it heat basically. As you just replaced the engine check for an exaust leak prior to the converter or loose O2 sensors. I'll send the thingsto check for. Otherwise you might have coolant leakinginto the converter.

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Saturday, April 16th, 2011 AT 11:50 PM

LAKEWOOD

MEMBER
12 POSTS

Thanks but I also could really use to know 1)Is bank 2 the one near the radiator (front) or the firewall (back)? 2) It appears this could be either the cat or a sensor - if so would it be the pre-cat or main cat? 3) If the sensor would it be the sensor in the manifold before the pre-cat or the one after the pre-cat (prior to the main cat)?

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Sunday, April 17th, 2011 AT 3:36 AM

HMAC300

MECHANIC
48,601 POSTS

It's the left bank which is commonly called bank 1 and it is the converter closest to the engine, meaning off of your manifold. That is the 3 way converter. Check for an exaust leak as you have replaced the engine or the O2 sensor leaking, etc. It is before teh converter. Also you might have an intake leak if your intake leaks coolant into the engine that will cool the convrter down causing this code.

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Sunday, April 17th, 2011 AT 12:54 PM