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Old 07-24-2012, 09:17 AM   #1
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Default Cat or O2 Sensor - Help Diagnosing

So I've got a 2004 CTS-V that I just bought and now every time it rains, the CEL light comes on for P0420 which is bank 1 cat low efficiency. The first time it rained while the car was parked and the CEL came on while driving home, and today it came on while it was raining on my way into work. Seems interesting that it rained both times.

I don't intend to tune the car so i'd like to avoid tuning out the CEL for the cat - so how do I determine if the cat is bad or if the downstream O2 sensor is bad without just throwing parts at it?

Both seem likely since water on a cat will reduce its efficiency and cause the CEL, but if I got moisture into the downstream O2 sensor connection, it seems plausible that that could cause the CEL as well.

Anyone have any advice? Know any place other than GMpartsdirect for a stock replacement cat? I dont want aftermarket hi-flows because I think theyll just throw codes as well on my non-tuned engine.

Thanks a lot
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Old 07-25-2012, 07:01 AM   #2
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This doesn't necessarily mean that your CAT is bad.

Have you done any major work to the engine in the past year or two. For example, using the wrong type of RTV sealant, etc. can cause the O2 sensors to go bad.

I'd start by checking the wave forms of the O2 sensors first. If they are bad and "lying" to the car's computer, then it would be a real shame to spend all that money on a Cat. The catch is that you'll need to find a sensor that checks the wave forms and have someone who knows how to do it. (A lot of mechanics will just want to sell you a Cat.)

Are you getting any other codes or seeing any other problems. Lot's of other things can cause this issue: http://www.obd-codes.com/p0420

To check the Cat, I'd try shaking the exhaust pipes. The Cat is filled with a fragile ceramic substrate that can break. Once it does, air flow is cut and it won't work properly. Here's what the inside looks like: (Air flows through all the small tubes that run the length of the substrate.)

Click the image to open in full size.
If the substrate is broken, you may hear a rattle.

I'd also look for signs that someone may have tried to steal your Cat. Such an event or even hitting road debris can do damage to it.
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Old 07-25-2012, 09:37 AM   #3
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Interesting that this only happens in the rain. I wonder if moisture is getting somewhere that it shouldn't?

But as I stated in your other thread, this code is triggered by a rear O2 response time and reading that is too quick and too similar to the front O2 reading when A/F changes are commanded by the PCM. An overly quick response time indicates reduced oxygen storage capacity of the cat. This code does not directly indicate a clogged nor physically damaged cat (though it can be the result of one), just an inefficient one - which can be a simple case of certain coatings wearing off the cat surface.

There are other ways to check for actual physical damage or clogging, such as looking for impact damage, or shaking/jarring the unit (as mentioned above) to check for loose material, or using an IR gun to check surface temps. If there is clogging you should also notice reduction in power, MPG, and driveability issues.
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Old 07-25-2012, 09:53 AM   #4
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Other than the cel, it runs just fine. No other work had been done to it and it is all stock. The cats look good from the bottom (no damage or dents), but i haven't rattled them around yet.

Tomorrow i am going to swap the down stream sensors and reset the light. If it comes back on the same side I'll swap the up streams and if it comes back on the same side again I'll replace the cat. Cheapest way i can think of to troubleshoot it without paying a shop to look at it on a scope.
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Old 07-25-2012, 11:26 AM   #5
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Quote:
Originally Posted by Bacon View Post
Other than the cel, it runs just fine. No other work had been done to it and it is all stock. The cats look good from the bottom (no damage or dents), but i haven't rattled them around yet.

Tomorrow i am going to swap the down stream sensors and reset the light. If it comes back on the same side I'll swap the up streams and if it comes back on the same side again I'll replace the cat. Cheapest way i can think of to troubleshoot it without paying a shop to look at it on a scope.
Good idea. Don't forget to use anti-seize compound on the threads when you do the switch. (I have deep personal scars relating to seized O2 threads - a "mechanic" did it, not me - and I don't wish the same on anybody!)
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Old 07-25-2012, 12:12 PM   #6
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Yea, that doesnt sound like fun. Im hoping that mine arent seized in there when I go to do the swap. When I bought the car (couple weeks ago) they said they just replaced the MAF - its a small town and Im wondering if they replaced this as an attempt to fix the CEL that they didnt tell me they had.

I think Ill pick some of this up on the way home:

http://www.wd40specialist.com/products/penetrating-oil/

Im assuming ill need it if I need to replace the cats - those flange bolts are usually rusted solid. They used this stuff on a rusted manifold on PowerblockTV and it looked pretty convincing. Worth a try anyway
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Old 07-25-2012, 03:34 PM   #7
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Quote:
Originally Posted by Bacon View Post
I think Ill pick some of this up on the way home:

http://www.wd40specialist.com/products/penetrating-oil/

Im assuming ill need it if I need to replace the cats - those flange bolts are usually rusted solid. They used this stuff on a rusted manifold on PowerblockTV and it looked pretty convincing. Worth a try anyway
Please just use that stuff on rusty bolts. It will wreak havoc on other components if you try to use it like a normal oil - which its not! (But it is good for rusty bolts.)


Quote:
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Yea, that doesnt sound like fun. Im hoping that mine arent seized in there when I go to do the swap.
No worries unless someone has messed with them before. (In my case, a "mechanic" took the sensor out and didn't put compound on it after.) The sensors come with anti-sieze already applied. If you aren't familiar with it, the compound is a paste with microscopic glass beads in it, which lubricates the threads and keeps the threads from fusing to the sensor bung under heat. Once the car heats up, that paste burns off and the glass beads fall out when you remove the sensor. So, you just need to re-apply that compound before you re-install a sensor that has previously been on the car.

Quote:
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When I bought the car (couple weeks ago) they said they just replaced the MAF - its a small town and Im wondering if they replaced this as an attempt to fix the CEL that they didnt tell me they had.
Maybe the previous owner sold it out of frustration? (I almost did the same before I discovered the MAF thing for my particular situation. BTW - K&N no longer gets my business.)

Interesting that the MAF was replaced. Are you sure its the right part?

The fact that someone has been messing with the MAF makes this more interesting. The fact that the car is new to you makes this challenge more difficult to narrow down. I'd definitely take your time figuring this one out before you plunk down for a new Cat. Just looking inside the converter (if you can undo that flange) may be telling. If its broken, you may be able to confirm that by holding it up to a light and seeing if the passages are still clear.

RPM WS6 also has a great idea regarding using an infrared thermometer to check out the Cats and compare them. If you don't have one, this would be a great excuse to get a new tool. They are really useful.
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Old 07-25-2012, 05:32 PM   #8
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I guess I don't see how the MAF could have anything to do with this code - isn't this code specifically tied to a comparison of the two sensors? I guess I never looked close enough at the MAF to see if it was new, but they said it was $600 so it must have been the MAF.
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Old 07-25-2012, 06:17 PM   #9
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Quote:
Originally Posted by wssix99 View Post
Maybe the previous owner sold it out of frustration? (I almost did the same before I discovered the MAF thing for my particular situation. BTW - K&N no longer gets my business.)


I agree. I don't use K&N type filters on MAF-equipped cars for this same reason.

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I guess I don't see how the MAF could have anything to do with this code - isn't this code specifically tied to a comparison of the two sensors?
Yes, this code is directly related to a comparison of the front and rear sensor readings when A/F changes are commanded by the PCM. It indicates a reduced oxygen storage capacity of the cat. I've seen MAF issues cause several problems in the past, but this has never been one of them.

In your case, either the cat is truly inefficient (possibily broken or melted material due to impact damage or misfire/overly rich exhaust overheating the cat, or possibily just coatings that have worn off causing an emissions increase with no other ill side effects), or there is an issue with the O2 sensor/wiring causing this code (less likely, but some have found this to be the case in the past...all you can do to test this is to move the sensors around like you suggested), or there is a problem with the PCM (much less likely, but still possibile).
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Old 07-25-2012, 06:23 PM   #10
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I appreciate all the responses - i plan on doing some work tomorrow evening and will report back. I guess if they had replaced the MAF, they could have had a rich condition that could have fouled the O2s and/or the cat....
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Old 07-26-2012, 10:18 PM   #11
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I have a feeling that posting this will cause bad luck...but I THINK I found out what has been tripping the CEL for the P0420 code. I got the car on ramps and rolled underneath with intentions of swapping the downstream O2s, and this is what I found on the bank 1 side:

This is a picture of the O2 sensor lead - note the drops of water on it hours after driving on a dry day

Click the image to open in full size.


Now here is a picture of the O2 sensor connector - note the water on the bottom of the connector:

Click the image to open in full size.


Now look up behind the connector at that black "duckbill." It has a drip of water on it:

Click the image to open in full size.


So naturally I thought to myself...wtf? Why is water coming from near the gas pedal? Went inside and pulled the carpet off the bell housing and found this:

Click the image to open in full size.


So apparently GM put a drain in the HVAC system so that when its hot and humid, the condensation can pee out and not leave a stink in your vents. Just so happens that the vent drains directly onto a wire loom and runs down into the O2 sensor connector. Enough water gets on it that it actually came out the other side of the water proof connector. So, I pulled it away from its mount, blew the water out and zip-tied it to a free hole on the transmission which is out of the drip path of this drain. I HOPE this fixes it. If not, the 02 may be toast from shorting out. Ill keep everyone posted, but if you have a P0420 code, check for this! Makes sense why the light would come on during hot hot days when the AC was on. Bravo GM.

The new layout - I also wrapped the back with electrical tape:

Click the image to open in full size.
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Old 07-26-2012, 11:48 PM   #12
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Those weatherpack connectors are designed to be water tight, so getting wet should not be a problem for them at all. But if the rubber seal of the connector was pinched or damaged, then water could certainly find a leaking point. If this was the case though, I'd expect several other O2 related codes on that sensor due to short circuits.
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Old 07-27-2012, 06:00 AM   #13
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A theory for why the issue in the rain. If the cats aren't working at full efficiency to start with, they may be more sensitive to temperature, and the rain cools them to much to be in their efficiency range.

I could be out of line here, but just a thought.
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Old 07-27-2012, 08:01 AM   #14
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I stopped at the auto store on the way to work this morning and cleared the code. While I was doing that I checked everything else from the FF and it looked normal. Both STFTs were -3s and the LTFTs were ~.7. I didnt see anything out of the ordinary, but it didnt have any measurements of the sensor voltage at the time of failure.

I drove around for about 20 min to make sure it had time to run the diagnostic again (min 10 minutes running) and then sat at a couple long lights and again in the parking lot to give it the idle period it needed. No light yet, but that doesnt mean anything. It could very well be a marginal cat, but ill try all these fixes first to save some money.
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Old 08-01-2012, 11:18 AM   #15
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Update -

Light came back, the wet connector was too good to be true. I did get the light to go off and stay off though by spacing the O2 sensor out with a spark plug non-fouler. I know its not "fixed" but it helps confirm that the sensors are working and that the cat is in fact bad (or marginal). Thanks to a generous member here, I have a used set of cats coming that I will swap in in the near future. They have similar miles on them, but they were supposedly working properly when removed. Since my car had a bad MAF recently, it could have caused a rich condition that fouled the cats in my car.

Ill update again when I get the swap done in a couple weeks.

Thanks to Soulja for the cats!
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Old 08-01-2012, 11:18 AM
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