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:
https://www.miataturbo.net/attachmen...ine=1348348997

Idle after 20 minutes of highway driving:
https://www.miataturbo.net/attachmen...ine=1348348997

Log during highway driving:
https://www.miataturbo.net/attachmen...ine=1348348997

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?

I had some of this fun with my 99 over the past year.


https://www.miataturbo.net/general-m...ank-1-a-61081/

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.

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! :vash:

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?

Age, burning oil, or excessively rich mixture over time.

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?

if bank 1 is low, then maybe you need a new precat?

Sounds like an opportunity to replace that boat anchor manifold with something better. Time to rejoice!

^^ 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.)