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Old 09-22-2012, 06:23 PM   #1
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Default P0421: Dead cat or bad rear O2 sensor?

I recently had a CEL with p0421 come on in my 37k mile CA-spec 1999 NB, which is usually the sign of a dead CA-spec precat. However, this is usually preceded by a p0300 code (indicates a bad coil pack), which I did not have. However, I also do not have any o2 sensor DTCs, so I really have no idea what would make the cat go bad in so few miles.

I tried to follow the advice here to determine whether the cat is bad or whether its a bad O2 sensor.

I tried to log my o2 sensors, and this is what I got.

Idle upon startup:


Idle after 20 minutes of highway driving:


Log during highway driving:


During idle, it seems to stabilize in the high range for a couple cycles, but on the highway it seems to stabilize in the low range, which is the opposite of what the advice above suggests.

Is my cat fried?
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P0421: Dead cat or bad rear O2 sensor?-5min.png   P0421: Dead cat or bad rear O2 sensor?-20min.png   P0421: Dead cat or bad rear O2 sensor?-highwaywarmedup.png  
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Old 09-22-2012, 06:56 PM   #2
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I had some of this fun with my 99 over the past year.


https://www.miataturbo.net/general-m...ank-1-a-61081/
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Old 09-22-2012, 07:03 PM   #3
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Thanks, I didn't see this thread!

This advice seems to coincide with what I'm seeing. I'm definitely getting about 50% inversions on my rear O2 sensor, so the cat is probably bad. I wonder why? Car is low mileage and hasnt thrown any misfire codes and doesnt seem to be running rich.
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Old 09-22-2012, 08:26 PM   #4
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Those o2s waveforms look fine to me, nice and even with fast response time.

P0300 is a random misfire code btw.
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Old 09-22-2012, 09:30 PM   #5
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Quote:
Originally Posted by flounder View Post
Those o2s waveforms look fine to me, nice and even with fast response time.

P0300 is a random misfire code btw.
Yeah, I havent gotten any other DTC's, so I don't know what caused the cat to fail. The car only has 37k!

Anyways, I checked the FSM, which stated that the O2 sensors should be "Approximately 15.7 ohms". I checked mine and they were 16.5 and 16.6 ohm F/R. Probably not statistically significant.

Alright, so what do you think could have caused the cat to fail?
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Old 09-22-2012, 09:49 PM   #6
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Age, burning oil, or excessively rich mixture over time.
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Old 09-25-2012, 04:46 PM   #7
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I just picked up a brand new Bosch 15330 ("OEM fit") O2 sensor, and its resistance from pins B and C is 8ohms, only half the resistance of my old sensors and what the FSM says...

Does this seem right?
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Old 09-25-2012, 04:52 PM   #8
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if bank 1 is low, then maybe you need a new precat?
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Old 09-25-2012, 06:04 PM   #9
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Sounds like an opportunity to replace that boat anchor manifold with something better. Time to rejoice!
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Old 09-25-2012, 06:05 PM   #10
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^^ Sorry, I wasnt clear. I'm 99% certain the precat is dead already. I had previously checked both F/R O2 sensors and they were both about the same resistance.

As preventative maintenance, I decided to swap the front O2 sensor. I ordered a replacement Bosch "OEM Fit" sensor, and it just arrived today. Before I installed it, I checked the resistance across the header leads, and it's half of what Mazda specs.

I was wondering if this is a problem. From an electrical standpoint, it just seems like lower resistance means it will heat much faster (it gets a constant 12V, which means the current must double, which means the total power dissipation is 4x as much as the OEM one.)
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