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2003 Mazda MPV Running Rough CODES

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Aug 16, 2011, 10:07 AM

Post #1 of 2 (6527 views)
2003 Mazda MPV Running Rough CODES Sign In

3.0L - V6
187,500 Miles

Ok I have 2003 MPV and it is running rough. The engine light came on but has been running ok until a couple days ago. The Mileage went in the dumper a quite awhile ago. I bought a Code reader but I need a bit of help ciphering out what exactly they all mean.

I figure the P0421 is that the Catalytic converter may be bad.

the part I am not sure if is which O2 Sensors are which What does Bank 1 Sensor 1 mean? Is this the sensor front Radiator side in front of CAT or Back Side front of CAT.

Also is Bank 2 Behind CAT?

I want to make sure I get the correct parts I need to fix the problem.

Warm-up Catalyst Efficiency Below Normal (Bank 1)
Pending Codes
HO2S-11 (Bank 1 Sensor 1) Circuit Low Input (No Inversion)
HO2S-21 (Bank 2 Sensor 1) Circuit Malfunction
Multiple Misfire Detected

Now I have one other Question.
If I have to order a CATALYTIC Converter is there only one on the 03 MPV and if so is the 3 bolt Inlet 2 bolt outlet bolt in model the correct one for my application?

Thanks 1000 times over in advance for any help.

Todd Fiebranz

Links from MPV forum removed

(This post was edited by Hammer Time on Aug 16, 2011, 11:13 AM)

Ultimate Carjunky / Moderator
Discretesignals profile image

Aug 16, 2011, 4:39 PM

Post #2 of 2 (6500 views)
Re: 2003 Mazda MPV Running Rough CODES Sign In

First thing you need to figure out, before buying a catalyst, is why the P0300 is showing. If you have misfiring going on, you need to determine which cylinders are misfiring. Once you determine which cylinders are missing, you need to figure out why. You'll wreck your new cat if you install it on an engine that is misfiring.

You have 02 sensors that are setting circuit and inversion codes too that can cause the computer to think the catalyst is bad when it actually isn't. You need to monitor the O2 sensor voltages to see what is going on using a scan tool or code reader with data stream capability.

Once you solve your misfire problem and O2 codes, then test the catalyst to see if it is good.

Don't just go replacing the O2s either. Inspect the harnesses for chaffing or burning on something. Check heater voltages, O2 heater resistances, and perform voltage swing tests using manufacture trouble charts before condemning O2s.

Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Aug 16, 2011, 4:49 PM)


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