Both P0420 and P0430 codes

[Brooke]

Heya~

So it's that time of year when I have to get smogged, and I'm getting a check engine light indicating both P0420 and P0430 (P0430 Catalyst System Efficiency Below Threshold (Bank 2)/P0420 Catalyst System Efficiency Below Threshold (Bank 1)) at the same time.

I'm driving a 2006 Off-Road Xterra. 198k Miles. I replaced the timing chain, water pump, all hoses, spark plugs, gaskets, O2 sensors, and the faulty catalytic converter on the driver side back in August. I'm having a hard time believing that both my cats are bad.

Does anyone have an idea of what could be causing this? I'm thinking the only thing left to check are the exhaust headers and maybe the piston rings?

Thanks in advance!

 

[Delete Me]

Sure. Think about those O2 sensors and how they work. The pre CAT sensor watches to see the voltage sweeping up and down. This shows times of rich oxygen and times of low oxygen and is done by the computer I think. The gas enters the CAT and the oxygen is consumed by the chem process. The vehicle knows this because the post CAT O2 sensor will read a flat line low voltage meaning ... no oxygen or little oxygen found in the gas leaving the CAT. This is how efficient the CAT is working.

Possibilites: (1) bad CAT (2) bad O2 sensor (3) cracks in the exhaust headers allowing more air ( oxygen ) to enter the CAT (4) hmm .. not sure. Something like bad gas, like diesel fuel contamination, can take out the CAT converters.

I personally would find a buddy that can look at the O2 sensor output to see how they are working but I'd probably take a quick look at the exhaust headers for cracks.

 

[Brooke]

Hi there! Thanks for the response. I've been monitoring the voltage on all 4 O2 sensors and I'm seeing bank 1, sensors 1 & 2 sit around 0.6v - sensor 2 on bank one will jump up to 0.7-0.8v occasionally. On bank 2, I'm seeing 0.6v @ sensor 1 and 0.8v @ sensor 2. Does that help?

 

[Beee]

Hi Brooke I feel your pain here. I have an 03 4Runner with the P0430 code too. Have covered many of the possible causes vacuum leaks, faulty O2 sensors and had the Cat replaced for $1100 then 1500 miles later the P0430 code came back. Not sure what to do next.


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[Zerobird]

Try clearing the codes? See if they return. Just cat efficiency codes, not gonna cause drivability issues.

 

[Arcanus]

Without actually seeing the voltage myself, it sounds like I would be looking closer at both of your sensor 1s. as long as it isn't one of those newfangled digital sensors, I would expect to see a sweep from .2v to .9v, with it usually sitting somewhere in the middle of that. this is the normal operating range of upstream O2 sensors. It shouldn't change much at idle or constant rpm, but as rpm and engine load goes up and down, the voltage should change as well. you seem to be implying that your sensor 2s are fluctuating more than your sensor 1s. This generally should not be the case. I would expect a more consistent voltage from the downstream sensors, since the cats will reduce the spikes in the exhaust stream.

The usual naming scheme is that the sensor 1s should be upstream, or "pre-cat" sensors. Down-stream, or post-cat sensors will be sensor 2. The bank# will tell you which side of the engine the sensor is on, with bank1 usually being the bank that has the #1 cylinder, bank2 is the other one.

You can't necessarily tell the sensor is bad just by reading with a voltmeter or low-end scan tool. The normal tool for diagnosis of O2 sensors is an oscilloscope or a high-end scan tool, but you likely won't be buying one of those to solve this problem, and any advice I can give you about how to use it is probably worth less than you paid for it.

 

[Arcanus]

I'm assuming this thing has some kind of exhaust crossover before the cats, as almost all modern cars do.

If one or both of your upstream sensors are bad, and they failed in such a way that it isn't obvious to the computer, all it will know is that its readings from the downstream sensors don't line up with its expectation, hence the cat codes, rather than upstream O2 codes. again, this is the kind of failure you really need an oscilloscope or high-end scan tool to find any direct evidence of.

If you say you have replaced the O2 sensors before, I am thinking one of 3 things:
1: one or more of the new sensors is bad.
2: they weren't bad, but somehow got fouled when they were installed
3: there is something else wrong with the vehicle that is either fouling the sensor(s) or causing them to read funny.

If you bought name brand or OE sensors, I'd say 1 is not likely. If you bought the cheap house-brand sensors from O'Riley's, AutoZone, Advance, whatever store, it is much more likely, but still not very likely.

I cannot speak to 2, as I don't know who did the work, but touching the end of the sensor with bare hands, or with any manner of automotive fluids/greases can foul sensors during installation. This also includes anti-sieze one may have put on the threads.

If you trust the sensor you bought, and the person who installed it, I would start looking at 3. Coolant leaking into the cylinder, even in small amounts that don't yet show signs of failure, can foul sensors. A more likely problem is an exhaust leak somewhere. Exhaust manifold gasket, cracked manifold, bad donut gasket, exhaust rusted out, etc. Fatboytx was on to something, methinks

 

[F4T XJ]

1. Check the headers for cracks and leaks
2. Check for intake leaks.
3. Can you see the long term fuel trims what are they reading.

 

[TerryD]

@Brooke Xterra's are bad about the cats breaking up and ruining the engine. The catalyst material is sucked into the rear cylinders and eat them up. You'd do well to get this thing to a shop ASAP and get the primary cats replaced. A couple hundred bucks for cats is cheaper than a couple grand for a used engine!

 

[DanR]

Former Nissan tech here. The cats are bad. Sorry for the bad news. You may get those codes again if you get aftermarket cats.

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[Beee]

The muffler shop that was used to replace the Cat in my 4Runner definitely put in an aftermarket Cat. The code now only come back when driving around town. If I reset it before a long highway drive it will not come on until I’m back in town for 20 to 30 miles.


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[DanR]

The muffler shop that was used to replace the Cat in my 4Runner definitely put in an aftermarket Cat. The code now only come back when driving around town. If I reset it before a long highway drive it will not come on until I’m back in town for 20 to 30 miles.


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Beee for $1100 that should have been an OE converter. Aftermarket cats are cheap.

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