Help diagnosing P0420

Got an 01 Tacoma which is throwing a P0420. Upstream air fuel sensor reads stoichiometric plus or minus depending on the circumstances; this sensor is about 6 or 7 years old. Downstream O2 sensor is new and is reading .7 V plus or minus .05 V; this was replaced recently due to needing a new exhaust system and a replacement sensor was not expensive.  The P0420 code predates the replacement of this sensor.  Normally, I'd think the upstream is bad, but the readings I am getting indicate it is fine.  This leaves me at the catalytic convertor being below the efficiency threshold.  Truck has 330XXX miles on it and is 19 years old, seems entirely plausible that this part has simply lived it's useful life.  Logic here sound correct(-ish)?

Catalytic converters are a wear item especially at 300k, I know the few really high mileage ones I have pulled out are basically blown out. 

It needs a cat. P0420 is RARELY anything else.

Its pretty rare for a 420 not to mean your cat is worn out.

When you buy the cheapest aftermarket cat you can find, you will probably still have a 420.

Appreciate the confirmation. Not buying OE, not buying the cheapest out there either. I can get the Magnaflow federal emissions OE style cat for around $260 which doesn't seem to bad. On one hand I hate throwing more money at this truck, but it has shown no signs of giving up the ghost. Ole' faithful seems perfectly content to move from point A to point B getting poor gas mileage for a long time to come.

Do you NEED to replace the cat for inspection purposes?  As long as its not plugged, the truck will continue to run fine...

Original owner of a very rare poverty spec 4 door 2001 Tacoma, 2.7L. 

At 225K miles, got the dredded p0420.

• Changed both 02 sensors. Didn't help. 
• Anti fouler plug. Didn't help. 
• L shaped bung to put the downstream 02 reading further out - LIke I use on my Catless Elise to pass emissions. Didn't help. 

Drove it till 250k miles with CEL. 

Then sold the truck to a movie studio - who is using as their truck for "terrorist shoots."  Had to smog the truck. Which was a big problem. My dad who had the same truck did an aftermarket CAT - didn't work. Need OEM CAT to really get rid of the CEL. 

After some finaggling, got the truck to pass emissions. Sold it. 

Yours is definitely the CAT. Depends on how long you want to keep it, emissions requirement, etc, and I would then decide on how much $ I would want to spend. 

My daughters triggered st 210,000 on her Civic.  YouTube suggests for off-road to put an anti fouler plug on the down stream.  
 

hmmmmmm

I got a anti fowler in a few cars and keeps the light off in all of them. 

No inspections or emissions testing in my area. I'm not necessarily a tree-hugging hippie type, but also don't want to pollute more than I have to. The trucks a keeper for using for truck stuff and occasional driver to work/backup vehicle duty.  The Torque app dyno function isn't the most precise thing but it reflects a pretty low hp number and the truck definitely struggles over 3-3.5K RPM, just makes more noise doesn't necessarily go any faster.

OBD2 codes are like clues in a scooby-doo mystery.  Except P0420.  That one is pretty clear.  Catalyst operating below threshhold.  On rare occasions, it can be the downstream O2, but yours looks to be reading properly IIRC.

Time for a new kitty cat.

I am a tree hugger, but I won't judge.  You could do an anti-fouler.  If that works, you could take the cat off and gut it and re-install.

But, universal catalyst cans are cheap.

Rock Auto sells a direct replacement Walker cat for $150.  Looks like a bolt-in with flanges.

I had a 420 on my Civic at 260 K miles. I bought one from Mr. Converter on Ebay for 99.95 including shipping. Works great and I sold my old converter to the scrap yard for 50 bucks. Why mess around with anti fowlers.

Mark L

As pointed out before "direct replacement" catalysts are not nearly the same as the original- which will likely result in a P0420 code very quickly.  Which would waste the effort to doing the change in the first place.

If one wants to take the risk, go for it.  As long as you realize that there are drawbacks.

BTW, on a side note- make sure that your car is burning a little oil as possible- that's the leading cause of catalyst poisoning these days.  Overheating is pretty rare for modern cars (misfire detection FTW), so other than just age and barring a mechanical impact- oil is the next problem.

It's not rocket surgery.  I've replaced hundreds of cats with Walker or other aftermarket cats at the shop and never had an issue.  At least no one came back a week later and complained that the light was still on.  I can understand if it's a Mercedes or BMW.  They get pretty specific with how they monitor their unobtainium parts, but we're talking about a Yota.

Agreed on the oil burning, but we're talking about cats with 330k.  The fact that they've lasted this long is close to a miracle.

bigbrainonbrad said:

Got an 01 Tacoma which is throwing a P0420. Upstream air fuel sensor reads stoichiometric plus or minus depending on the circumstances; this sensor is about 6 or 7 years old. Downstream O2 sensor is new and is reading .7 V plus or minus .05 V; this was replaced recently due to needing a new exhaust system and a replacement sensor was not expensive.  The P0420 code predates the replacement of this sensor.  Normally, I'd think the upstream is bad, but the readings I am getting indicate it is fine.  This leaves me at the catalytic convertor being below the efficiency threshold.  Truck has 330XXX miles on it and is 19 years old, seems entirely plausible that this part has simply lived it's useful life.  Logic here sound correct(-ish)?

Old front O2 plus new rear O2 will often make a false catalyst inefficiency code.  The sensors get lazy and slow to respond, when this happens at the front, it affects catalyst function.  You cannot judge front O2 function with a scan tool or a volt meter, you need an oscilliscope.  The switching rate is (should be) far faster than any scan tool's data rate, or a voltmeter's.

If you have to replace the rear O2, replace the front O2 at the same time.

Please do not install a replacement cat before you replace the upstream oxygen sensor.  Aftermarket cats, especially cheap ones, are often barely good enough to pass the monitor, and oftentimes they are not even enough for that.  Subarus especially are bad for needing an OE converter in order to pass.  So, if you replace the converter, you may be setting yourself up for more headaches unless you get a dealer part.  (Yes it may cost $800-2000 or so)

Mazdaspeed Miatas are very sensitive to sub-OE quality aftermarket cats. It's not just Mercedes. It won't throw a code that week, but any degradation of the cat at all will throw a code. FYI. 
 

A bad coil pack will kill a cat pretty fast. Or more accurately, misfires. That raw fuel will take the cat out in days. We see this a lot in 1999-00 Miatas, they lose a coil, develop a misfire and then the 0420 comes along right away. 

In reply to Keith Tanner :

If they are not detecting a misfire due to a coil fault, that's a serious problem.  As I see it, at least.  Probably not a problem in the OP car.

In reply to Pete. (l33t FS) :

Most switching is in the 1hz range.  Should be able to see that with a scan tool at 10hz.   Then again, I can't remember how fast the SCP OBD systems outputted the data.

alfadriver (Forum Supporter) said:

In reply to Keith Tanner :

If they are not detecting a misfire due to a coil fault, that's a serious problem.  As I see it, at least.  Probably not a problem in the OP car.

Oh, the car detects it. But that doesn't stop the owner from driving it. When someone calls in with the misfire code (0300 or something like that) you know they'll follow up with 0420. After all, everyone knows you just need to tighten the gas cap and then you ask a forum why your car is running badly before you go to the trouble of stopping by Autozone. 

alfadriver (Forum Supporter) said:

In reply to Pete. (l33t FS) :

Most switching is in the 1hz range.  Should be able to see that with a scan tool at 10hz.   Then again, I can't remember how fast the SCP OBD systems outputted the data.

I've never seen any scan tool refresh at 10hz, it's always waaay slower than that.  I've also only been able to use aftermarket devices, and rumor has it that the OEMs keep backdoors to fast data all to themselves.

1hz sounds really low compared to what I've seen, for a system with a modern computer and heated O2.  I've seen old one wire systems with slow clunky computers swinging that slowly, but I know GM at least is able to look at individual exhaust pulses to monitor cylinder to cylinder mixture variation on the same bank, so I'd assume that an OBD-II Nissan is a lot closer to that than KE-Jetronic.

I wonder if the obd2 code team was laughing the whole time they decided 0420 was "catalyst operating below threshold" because I know people who go 0420 regularly and they operate well below threshold 

Since you all have kept this going, the 0420 predates replacement of the rear O2 sensor. That sensor as well as the exhaust was replaced due to numerous leaks, rot, and the studs for the sensor literally disentagrating when I put a little torque on them.  Noted on throwing a fresh front sensor at it before spending real dollars on cats.

Magnaflow is one of the more reliable and quality aftermarket catalytic converters correct?

Pete. (l33t FS) said:

alfadriver (Forum Supporter) said:

In reply to Pete. (l33t FS) :

Most switching is in the 1hz range.  Should be able to see that with a scan tool at 10hz.   Then again, I can't remember how fast the SCP OBD systems outputted the data.

I've never seen any scan tool refresh at 10hz, it's always waaay slower than that.  I've also only been able to use aftermarket devices, and rumor has it that the OEMs keep backdoors to fast data all to themselves.

 

1hz sounds really low compared to what I've seen, for a system with a modern computer and heated O2.  I've seen old one wire systems with slow clunky computers swinging that slowly, but I know GM at least is able to look at individual exhaust pulses to monitor cylinder to cylinder mixture variation on the same bank, so I'd assume that an OBD-II Nissan is a lot closer to that than KE-Jetronic.

Just because you look at it does not mean that it's part of the closed loop cycle to run the catalyst the best.   As sensors got better, control got better, and the ability to tweak how it switches for best catalyst performance got better.  By the OBDII era, we all has the rough ability to run the system at a pretty specific frequency for catalyst performance- which does come out to the 1hz range.

The faster reaction is more about dealing with a specific a/f control issue just to keep the control so that one can run it at best catalyst performance.

BTW, for the 1hz range, that's 2hz to .4 hz- or a cycle of .5 sec up to 2.5.  10 hz is just not really great for catalyst effiency- tried it enough to know at too fast of a speed that it really just is noise to the catalyst when the goal is to cycle the state of it back and forth for good balance of efficiency.  At some point, too fast also means you are not getting much an a/f magnitude change- which just make the a/f look like noise.  Which is also hard to get good efficiency.  

All that to say- faster isn't better, so I know they are not actually intentionally switching that fast.