Thread: Random PO420 Code

Hey guys. Car has been doing great, but today while running about 80mph on the interstate today I looked down and the SES light was on. So I took it by Autozone to get it scanned and they told me it was throwing code PO420.
"Catalyst efficiency low-bank 1"

From searching on here, I've found bad cat, clogged cat, bad o2. Which one is it?
I have a stock GT with 158k, 114k on the engine.

Any ideas?? Is it fine to drive for now?

Thanks!

Originally Posted by SlowNA06

I wouldn't be surprised by a clogged cat at that mileage.

It's totally unsafe to drive with one, though.

hm. well if it's clogged does that mean you have to buy a new one?

and was that sarcastic? because i really need to keep driving my car..

Driving hard with a clogged cat is a good way to blow up your engine. Not sarcastic. The cat (catalytic converter) occurs right before the rear O2 sensor, and just after the front O2 sensor in your exhaust. If it is clogged, then exhaust gasses can not pass easily, which will increase backpressure, and increase knock to an unsafe level where you'll chip a piston or something else uncool.

That being said, you're safe for a little until you replace it, since it may not be completely clogged - or particularly clogged at all. The P0420 code just indicates a drop in cat efficiency. A test I've seen used to test for a clogged cat is to drive around with a warm engine for a bit in the dark, park, and look at the cat to see if it's glowing red. You may also smell sulfur/rotten eggs from the exhaust.

From what I've seen, the life of a cat is only about 150,000 miles anyway. Buying aftermarket means you're paying $50-75 instead of a dealership's $600.

are most all aftermarket cats suitable to purchase and install or is there a particular brand?

Thank you for your advice

When my cat was going bad at around 116K miles, I started to notice loss of power that got progressively worse until I could barely make it up hills. Eventually it got so bad that when I looked under the hood in the dark, my rear exhaust manifold was glowing red. That's when I knew it was really time to change it. Luckily, my year GP GT was still under the 120K miles warranty and the dealer replaced it for free. After changing it, my car felt like a new car once again.

hm, well mine doesn't really bog. it's pretty damn peppy; i was rolling about 65-70 today and gave it about 75% throttle and the RPMs shot up to 5-6 and I didn't feel a bog. Granted I didn't feel a lot of pull either. I'm a GT still.
My front exhaust manifold was a little pinkish dusty upon looking after killing it.
IDK, I guess I'll just use christmas money and tax return money and get another cat put on because I don't want chip pistons I wanna keep my GP around for a while. Guess struts will have to wait.

I'm going to call the shop down the street tomorrow. What should I be looking to pay for the cat and to have it put on?

Oh and today is the first day the code came up so mine may not clogged enough to make a difference...yet. idk though i'm just guessing

Originally Posted by SlowNA06

The whole ZZP downpipe + cat setup that you can bolt in yourself would set you back about $120. So I'd look to pay no more than that.

okay. so this is what you are talking about?
if so, which size? and it would be $170 with a cat.
but just in case, i wouldn't have to weld anything right; just bolt in??

thanks!

edit: i called around muffler shops and the cheapest (and one i feel i can trust) said depending on which cat (fed vs. california specs or whatever) it would be $200 or $250 for parts and labor. does that sound about right? or do i need to purchase a cat on my own and have someone put it in? or do you guys have another alternative?

I'm sorry for bringing this old thread up, but it was the closest one to what I'm looking for and I did not want to start a new one.
I just learned a very important piece of information here, i.e. OEM CAT's (around $1100) is not much better than an after market one ($300 direct fit or $75 universal) and that's a lot of saving for me. I still do have a few questions though and I will put them in points below (easier to read than boring paragraphs, I guess).

1. Car I'm concerned about is an 08 grand prix with a V6 3800 series III engine that has about 158KMiles that had O2/CAT codes a couple of times last year that when deleted did not come back till now where for the last couple of weeks, PO420 comes back almost the next day whenever I delete it. The code means that CAT efficiency in "Bank One" is not good. I did some research and found that there is another code called PO430 for "Bank Two". My first question is, if my big V6 engine only has one upstream and one downstream sensor and one CAT after which the tube splits into two and each ends in two openings at either side of the rear (simply by looking under the car), then are there even bigger systems that utilize the PO430? Do they have two separate CAT's for the different banks of the engine and 4 sensors?

2. I used a Bluetooth OBD2 gizmo with my phone attached to the dash displaying the wave-forms of the upstream and downstream O2 sensors simultaneously for many days staring at those wave-forms as much as safely possible while driving. The upstream is oscillating as it should but it looks closer to random than I thought. The amplitude may range from 0.1-0.6 or 0.3-0.8 or even sometimes missing a bounce up or down and it looked like pressing the pedal increases the peak-to-peak bounce to more like 0.1-o.9V. The downstream in more or less constant at close to 0.7V but sometimes (rarely) does bounce but with a much lower overall change than the upstream. They both start at about 0.45V bias when starting from a cold engine, but there is a random point in time that triggers the code when the engine warms up after driving for a while where the upstream signal keeps bouncing but at about 4 times slower and at a wide range (probably 0.1-0.9V) while the downstream shows almost the same shape but a tiny little bit lagging. That code triggering behavior only lasts for less than 5 seconds and the waveforms go back to normal (so if i delete the code, it may never happen again for hours). Does my description show any sign of something wrong other than the CAT?

3. I read here that CAT's last about 150K miles (I'm sure that the writer has that theory from a long experience and that it is a rough estimate that could be more or less), and that the car felt like new after replacing it, and then a local NAPA guy told me yesterday that it is a common practice to replace both sensors when replacing the CAT because the almost always age together. Considering the age of this car, is it crazy if I just go ahead and replace all of them without real and extensive diagnosis (that may cost more)? By "extensive testing", I mean, some of the crazy things that people with $10K snap-on scanners do, like propane test for O2 sensor reactions, or smoke machine for vacuum leaks or advanced O2 sensor wiring tracing.

4. OEM or cheap? Direct-fit or universal?
note: both O2 sensors are $32, while AC-Delco (made in China) is $120.
Is this whole job a DIY-friendly one? and does installing universal cost a lot more than direct-fit?

It is an aging beauty, so spending a buck less on her is a great thing for my current financial situation, and any help by advice or opinion is greatly appreciated.

1. Yes, vehicles with 2 cats and 4 o2 sensors would use the other code P0430. Like the later model 3.6 impala's or something like a silverado truck with 2/3 cats.

2. The job of the upstream narrow band o2 sensor is to bounce lean and rich of stoich, where stoich is 450mv give or take. It's taking real time readings and doing on the fly fueling adjustments based on what it see's in the exhaust gases. The up and down motion of the data logs should be pretty constant, like what you are seeing. That .200-.800mv with some random .100 and .900's depending on throttle input and fueling needs of the engine. The more throttle you give the faster the changes go and range they go increases if that makes senses. You kind of already descibed that, the gap between the switches increases because more fuel and oxygen are inside engine and exhaust gases. Als if like DFCO (decel fuel cut off) was active the o2 mv's tend to dive bomb to .1 and when you are heavy into the throttle they will stay above .800-.900mv.

The rear o2 sensors tend to switch much less as the exhaust warms up and will level out like that if the cat converter is doing it's job correctly. Typical readings tend to be in that .550 to .700 range and pretty much stay there all the time. If the rear sensor starts to switch like the front sensor and the computer see's it more than a few times in a given amount of drive cycles it will trip the code.

As for the cold start mv readings are .450, it's mainly because the vehicles computer does not use the o2 sensors at all during cold starts. The computer does not care what the o2 sensors read under a certain coolant temp. There are tables inside the calibration that control what the fueling will be while the engine temp is colder than say 120-130 degrees. Typically a car will run around 12-13 AFR while it's warming up and switch to regular stoich ratio of 14.7 for regular gas once it reaches closed loop fueling at that 120-130º coolant temp.


3. Cat life is always a toss up, it's a mass produced part that they say should last well past the 100k mile warranty but you never know. Pre-mature failure happens and actual engine issues can shorten the life of the part. Bad fuels, poor maintence, tons of short trips that don't allow things to get fully heated inside the cat can shorten the life. Carbon build up from not beating on the engine a little could do it too. The 3800 isn't the a super clean engine and prehaps part of the reason they did away with it.


As far as cats and o2 sensors go. Buy decent stuff at least. Several people have bought the Walker Direct Replacement cats with zero issues, myself included there. Easy to bolt it and been trouble free for me. O2 sensors you don't want to cheap out on. I always buy OE parts, no off brand parts or bosch parts on a 3800. Only AC delco or Denso sensors, either work great. If your's have over 150k miles, you should replace them for sure. They have made it way further than they should. Typical replacement tends to be around 100-125k in most cases.

This may not have answered every question but mostly all of them.

Is it urgent, likely no in your case. It doesn't seem you have an major loss in power from a clogged converter or anything like that. If the code was constant you may notice a loss in fuel mileage though.

A univeral cat would require more labor, as it would have to be welded in and that takes time. It's likely less than 1hr labor to remove and replace the whole downpipe with converter. Heck if all the bolts came right out, less than 15min with a impact gun to do the whole thing.

theres nothing hard about getting to the up stream o2 at all. you need a o2 socket, then 12 inches worth of extensions and a ratchet or breaker bar, cause they get rusted in there. once it cracks free it comes right out. the socket cost 8 bucks at hf, change it your self. middle of the rear valve cover is a bracket with a plug on it, follow it down to the o2, its like right there in the rear manifold, cant miss it.

the down stream o2 likely will fight and not want to come out at all. shop with a torch when they do the cat might be best if you cant get it out.

napa has over head unlike most internet stores. stuff will cost more at the local parts store.

get delco o2's. why buy them later on when the denzos fail? wasted money, you get what you pay for. look on ebay for your o2, might find em cheaper. rockauto is usually pretty good on o2 prices as well.

if your down on power, engine sounds like a vacuum with a full bag....cats clogged, you can smash it with a hammer to free it up sometimes. if you hear it rattle its shot to hell.

you can also drive it at night spirited if you will, and then stop and check to see if the cat is glowing red.