Geesh now I have a P0421 code

[fortworthmatt]

So my check engine light came on the other day with the dreaded p0421 code. After reading the forum and checking my records I want to make sure I start off spending money in the right place. The previous owners had both cats replaced around 100k and car is currently at 145k. Last time coils were replaced was at 104k miles. I just replaced the spark plugs. So should I start with the coil packs after I am sure there is no exhaust leak? Car runs fine but does idle a little rough. I have also cleaned the egr valve. I know this is a sore subject but could use some advice.


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[kms1990]

my sisters 626 has that code come and go make sure your o2 sensors are good and clean (seafoam the engine) and look for exhaust leaks

 

[fortworthmatt]

Well I replaced my coil packs and wires and after driving a while this morning the CEL went off! Hooray!


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[fortworthmatt]

Oh wait now it's back.


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[31N007]

Have you looked into the possibility that it could be the secondary O2 sensor that is faulty? If it's bunged up or whatnot, that might mess with the reading that the computer sees. If you look at the reading that the sensor sees, it'll oscillate from one voltage to the next. If the sensor is coated in some crud, maybe the voltage won't fluctuate enough and then the computer throws the code.

The issue I'm having with my car is that the sensor will sometimes trip the code (did the other day) but it was fine for a month or so. I gutted the primary cat as it was crumbling and rattling around. This might have also had something to do with the fact that my coil packs went out.

I'm not sure whether there is a procedure for checking to see if the secondary O2 sensor is performing correctly, but you might try and have a look down that path before you look anywhere else. I would say it's safe to assume that the replacement cat hasn't gone out in such a short period of time, but if your car misfires, that might just be enough to cook the replacement.

 

[fortworthmatt]

Yeah I think that is my next step!


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[FlyinHawaiian]

Any updates? I'm throwing the same code

 

[MrGiggles]

The P0421 code is set when the post cat 02 sensor is mirroring the pre-cat 02, meaning that the cat isn't converting the exhaust gas at all. That is the only purpose of the post cat 02, to monitor catalyst function.

You can check the 02 with a scan tool or the Torque app and an ELM327 module.. If the pre cat and post cat 02 voltages switch in unison (should toggle between .1 and .9 volts) the pre-cat isn't working, and is the cause of your code. If the cat is working, the pre-cat sensor will toggle normally (will show an ocean wave form on a graph) and the post cat sensor will show a much straighter waveform in comparison.

There are a couple of other codes that will usually set with a faulty post cat 02 (P003X). If you are getting a P0421, there is a very high chance that the cat is the problem, not the 02 sensor. This code can be cured by the non-fouler modification.

 

[fortworthmatt]

I changed both O2 sensors and it went away.


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[MrGiggles]

I changed both O2 sensors and it went away.


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The catalyst monitor is one of the last to run, so don't be surprised if it shows up after you drive it for a couple of hours.

 

[fortworthmatt]

It was definately the O2 sensors. I drove it for a couple of weeks and all was fine. Even got the car inspected and car passed fine. Anyway I sold my P5 and bought a bigger wagon, a Volvo XC70.