2009 oxygen sensor problems new ecu and now p-0420 catalytic converter

Hello,


I've lurked on here for quite some time, using old threads to help troubleshoot and maintain my vehicle. Let me say thank you in advance for reading this post and for all of the knowledge and information that has been compiled here throughout the years. I am sorry for the length of this post, but I am trying to provide as much information about the current situation in hopes for a better understanding.


I have run into several problems on my automatic 2009 Aveo within the last year. I couldn't rid myself of codes 0141, 0140, 0138, 0036, 0053, and 0054. I installed a new Bosch upstream o2 sensor, then a NTK. I also installed a new NTK downstream o2 sensor, followed by a much cheaper Kwiksen brand sensor. All of the cel oxygen sensor related codes persisted. During this time I also installed new AC Delco spark plugs and found several holes in the mass airflow ducts. I unplugged the mass airflow sensor during this time and several mas related codes showed up. I plugged it back in, cleared the codes and thoroughly duct taped the holes after researching a replacement and finding that the whole OE system had be to bought. There was a small amount of oil pooled in one of the plug wells, but tightening up the head gasket appears to have taken care of that.


Previous to these repairs I had only replaced the coil pack after the "hold" light came on one night when driving home two years ago.


After checking to make sure there were no apparent shorts or breaks in the wires from the o2 sensors I tested the connectors with a digital multimeter. There were no problems with my findings.


At this point I took the vehicle to a local repair shop to get it diagnosed. I had an NTK o2 sensor in the upstream and the Kwiksen o2 sensor in the downstream. After several days they contacted me saying they were 99% sure that the ECU had to be replaced. According to them that would take care of all the codes that were present. It took them almost a week to replace the ECU. They had problems with the programming and only told me that they had to install an update to finally get to take. I don't know if the update was hardware or software related.


After the ECU was installed and programmed they told me that the downstream o2 sensor wasn't acting properly. I wasn't exactly surprised because the Kwiksen sensor was really cheap the connector fit seemed extremely tight. I agreed to have them replace it with an AC Delco downstream sensor.


I went to pick up the vehicle and they told me when they went to move it out of the bay that it threw a code for the upstream sensor. I brought them the original AC Delco sensor as well as the Bosch that I had purchased. They installed the original AC Delco sensor and the vehicle ran fine for a while then abruptly died and threw a code. They suggested installing a brand new AC Delco upstream sensor. I asked them about the Bosch that I had brought them and they hadn't even tried it. They installed it and the vehicle ran fine with no check engine lights.


While trying to complete the drive cycle so I could finally take the vehicle through emissions testing, the car threw an upstream o2 sensor code. I bought a brand new AC Delco upstream o2 sensor, installed it, and reset the cel. I drove around town for a while with no problems. The next day as I am sitting in the emissions testing line the cel comes on. I took it right back to the shop and after 2 days they called me and left a message with the info that it is p-0420 and related the the catalytic converter. They are going to try and charge me for another diagnosis now.


I really have no idea what to do now. I didn't have a chance to return their call before they closed for the day. I don't completely understand why only the OE o2 sensors will work in my vehicle. I really need to get this sorted out so I can get my tags renewed. Based on research it looks like the problem could be related still to the air / oxygen ratio or the sensors relaying info that is too similar? The only physical problem I have noticed is the vehicle running slightly rougher when the ac is turned on. I was planning on adding some freon after I got the tags renewed.


Any help would be greatly appreciated because at this point I am really at a loss.

Thanks in advance,
Donovan

The shop said the o2 sensors were functioning properly, and insisted on replacing the catalytic converter. He remarked that the cat was bad 3 times in the ten minutes it took him to track down all the paperwork for me to receive the vehicle back. After picking up the vehicle I started removing the front half of the exhaust system. I wasn't aware that the front cat was welded to the manifold. I removed it and did my best at visually inspecting it for any holes or noticeable damage. I let it soak in a water / degreaser bath for around 4 hours and then thoroughly rinsed it out. I finished reattaching the front half of the exhaust system earlier today and installed a spark plug non fouler on the downstream oxygen sensor. I realize the non fouler is just a band aid on what is probably a much larger problem, but my tags were due in November of last year. I reset the cel and drove it ten miles or so home via the interstate. There is no real difference in how the vehicle is running with the exception of initially starting it up to back it off the ramps. I have almost a full tank of gas, but I will probably feed it some premium on the next fill up. Depending on how the vehicle runs on my 50 mile trek to work and back tomorrow, I will consider some fuel additives / exhaust system cleaner when I next visit the auto parts store. If it throws another 420 after 70 miles, I will more than likely start messing around with the rear catalytic converter. I am really hoping that the recent work will at least get me through emissions inspection.

I am sorry I missed this thread the first time you posted.

Based on EVERYTHING you have done, I have a hard time believing the issue is anything that hasn't been addressed. And you have spent a lot of money from the sounds of it. (I would verify your duct tape job) and maybe even replace the pipe with a cold air intake pipe.. (it will work, ill get back to this).

I worry about the procedure you used for the cat. I have never heard of soaking the cat in water to clean it. As well as the suggestion of using higher grade fuel, or fuel additive. I don't believe either of these are necessary, nor will the benefit you. They can also worsen the problem.

Two things I have found with my car, NGK copper plugs are all I would run. They were used OEM, and eliminated a lot of issues. The only problem with them, they absolutely must be replaced @ 20k miles.

I had replaced both of my o2 sensors. Mine were under warranty when they went so I didn't use aftermarket. IIRC oem was the NGK? I cannot remember though. Anyhow, in my experience with most cars, if you do not use OEM you need to "match" brands or specs.. I always tell people to not buy the splice in o2 sensors to avoid getting messed up.

There is only one thing that kills CATs on a properly sealed engine (no oil of coolant getting in cylinder). Unburned fuel. This is where the repair job to your intake pipe comes in. Unmetered air entering the engine will cause the o2 to discover a lean condition and dump more fuel. If you are running high octane, and fuel additives this just adds to the problem. If you think of the engine ECU as operating a venn diagram where optimally all the sensors point to the same area = no codes, good running engine. Each sensor can be a little off and still in range. But they do effect other sensors. When you replace your sensors as they go bad, the union area of the venn diagram changes a little. Each sensor individually can be in their "good reading" zone, but the overlap (union) of the sensors could be off. This effects the fuel map of the ECU.

Anyhow, in my opinion. I do not know if your cat was bad. But you "cleaned" a clogged cat, a cat can also no longer efficiently oxidize particles. This is generally a hollowed out cat, where the ceramic core is missing. I do not know what you tried to fix.
Anyhow, I will try to explain what could be going on in my opinion, using some numbers.. think of the venn diagram.

The three sensors re primary o2, secondly o2, and the MAF. The common signal from these is around 0-5. Individually 0-5 is a good sensor. However, aftermarket parts tend to have a range "close enough" like 0.1-4.9, or -0.1-5.1. When everything is running perfect, all the sensors will be down the middle, or 3 by coding not by reading. However, if two sensors are running around the 5 mark, and the third is down by 1. That would be the sensor throwing the code. But it could be the issue of one of the other sensors staying in the 5 corner (if you look at a Venn diagram, 2 sensors can still be in union while the 3rd is not loner in the union, it can only adjust as far as its circle allows). There can be nothing wrong with that sensor, but that is the code you will get because it doesn't keep in range with the other two sensors. This is why I say fix the MAF/intake issue.

Onto the next possibility, the secondary o2 should "always be half" of the primary o2 to show cat efficiency, again in code not the voltage. But in live data, the voltage change should be parallel. Again, they can both be reading between that 0-5. But if the secondary o2 is only work 3-5 and the primary is sitting at 1, you will get a code. The primary o2 will never throw a code on a good running engine. The secondary will throw a code when it doesn't stay parallel to the primary. However, if the primary o2 is reading in a range to compensate for a lean condition, and the secondary cannot adjust, it will be the secondary that throws the code.

After a rather lengthy explanation, I would say put NGK coppers back in, and fix your maf/intake. (Buy a short ram intake on eBay for cheap) you could even get a flexible one from auto zone and keep the airbag if that is an issue). Run the 87 octane the car is deigned for, and do not use any fuel additives. After you do that, see what happens and post back.