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Old 04-02-2015, 11:18 AM   #1
havok
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Default P0420 Code Help

I have a 2007 Vue Greenline (which I love) that has 150,000 miles and has had all scheduled services done in a timely fashion. I had plugs and wires done at 100,000 miles plus a coolant flush and fill at the same time. Recently the car set code P0420 Catalyst System Efficiency Below Threshold. I was told that was the only code set. Left the car and they checked for exhaust leaks (none) and checked the O2 sensor responses with a scan tool. They didn't feel that the responses were out of line but I didn't push for the actual data. I've noticed little change in gas mileage. About 1,000 miles were driven before the CIL came back on and, again, P0420 was the only code set. We are leaning toward replacing the O2 sensors first as I've gathered from reading through the forums that it's not a bad idea to replace them when they get to be old. Otherwise, any ideas? I'm hesitant to just replace the cat as it's my understanding that, unless there are other issues, the cat should last longer than it has. Thanks for all help/advice/suggestions everyone here has always been quite helpful.

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Old 04-02-2015, 03:37 PM   #2
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Default Re: P0420 Code Help

At 150k miles, nothing lasts forever. Whether its a well worn catcon or rear O2 sensor, its a coin toss as both are well past their factory and emissions warranty making them candidates for replacement. The rear sensor measures catcon efficiency so its age and wear would have its own set of error codes. Here's a list from the service manual;

Probable Causes:
  1. Sensor or solenoid faulty
  2. Connector terminal contact is damaged or corroded
  3. Wire harness - Check harness for correct voltage, open, short to ground or short to voltage
  4. Update control unit software - Check for the latest control unit update
  5. Powertrain or Engine Control Module faulty
  6. HO2S sensor contamination/damaged
  7. Exhaust leaks or restrictions
  8. Catalytic converter damage
  9. Catalytic converter improper operation

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Old 04-03-2015, 02:07 PM   #3
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Default Re: P0420 Code Help

fdryer, thanks for your insights. I agree wholeheartedly with your assessment of "nothing lasts forever." I should have replaced the O2 sensors 70,000 miles ago. I was led to believe that catalytic converters "never die on their own." But, I took that with a grain of salt. Certainly if the cat and the O2 sensors have to be replaced, it stings but it's nothing to be upset about.

Given that there have been no other codes set and no other drivability issues, I'm hesitant to blame anything but the obvious Also, given that the code doesn't return for a few thousand miles when it is cleared, I'm also leaning toward a cat that is reaching the end of its useful life.

We changed the two O2 sensors today since we would replace them if we replaced the cat anyway. We'll see how that goes. If the code returns, I'll bit the bullet and replace the cat. To that end, has anyone had any luck with aftermarket cats for the hybrid Vue or should I stick with OEM?

Thanks again for the insights and opinions of a very talented group of people.

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Old 04-03-2015, 06:10 PM   #4
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Default Re: P0420 Code Help

If its of any benefit, I fought (actually reset several times long before NYS inspection was due) two error codes occurring about a year apart for two of four O2 sensors at the time when my car had about 75k miles. Each error code was for each front sensor in my V6. Each is a spendy replacement. To "add insult to injury" about replacing these O2 sensors.................the error codes were not about the actual sensors but for a faulty heater in each one!? Since I had time to 'play' in between NYS inspection, I recognized the heater error code and allowed the automatic OBD II system to reset the error code on its own. Being erratic and random when the heater error occurred, I thought it was related to salt (winter conditions) and observed each engine/drive cycle and tried manually resetting with a reader. The error code would randomly just come back on its own without noticing any change in engine operations or fuel economy. In the end, I replaced each front O2 sensor. The error codes never came back. 75k miles and both front O2 sensors needed replacement for faulty heaters..............

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Old 04-20-2015, 10:45 AM   #5
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Default Re: P0420 Code Help

So, after replacing the O2 sensors, the P0420 code has come back and gone away on its own. On the recommendation of my local GM service writer, I took the Vue to an exhaust specialist who believes that there is a crack in the exhaust manifold that is still causing the P0420 code and that there really isn't anything wrong with the cat. This sounds reasonable to me. How difficult is it to replace the exhaust manifold? Thanks.

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Old 04-20-2015, 02:00 PM   #6
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Default Re: P0420 Code Help

Whether its a faulty exhaust manifold or worn out catcon is for you and trusted repair shop to decide. How did the exhaust shop determine the manifold is faulty? This can be an expensive guessing game........ The exhaust manifold may be seen after removing the heat shield so checking for exhaust leaks can be done at home. Pm an email address for files on manifold replacement. Be sure to mention model and year.

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Old 04-20-2015, 02:18 PM   #7
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Default Re: P0420 Code Help

I hear you on that ... the exhaust "specialist" identified first by sound, then he visually pointed out to me that there is what appears to be a hairline crack at the bottom front of the exhaust manifold, almost dead center left-to-right. His assertion is that the crack goes all the way around and that there is a leak there. The garage the I've used for years supposedly checked for leaks but I'm not sure how thoroughly. I did point out to my regular shop that I thought I had noticed exhaust odor at the front of the car and in the passenger compartment when the heater was going. I guess we can't be 100% without removing that heat shield and then we'd no for sure. I guess I was expecting a failed cat to be a more "consistent" failure ... but, it could be just barely below efficiency so it takes 1,000 miles for the code to reset. Thanks again for all your help.

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Old 04-20-2015, 03:27 PM   #8
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Default Re: P0420 Code Help

OBD II systems automatically detect errors and either resets an error code on the next engine start or after three complete engine start/drive/engine shut downs. This is under ideal conditions presuming errors are followed up with correct repairs. A gas cap left loose and causing an error may take up to three drive cycles before the CEL turns off on its own. For you to see an intermittent repeating error code resetting and reappearing randomly means a problem persists and requires more than average troubleshooting. A leaking exhaust is on the list of possible faults and smelling exhaust under the hood as well as inside the car adds up to suspicions. It is unusual for an exhaust manifold to crack and if it turns out to be true, why did it fail?

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Old 07-16-2015, 10:49 AM   #9
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Default Re: P0420 Code Help

So, I have some more follow up information if someone can help interpret.

Just to refresh: I replaced both O2 sensors at the beginning of this ordeal.

The exhaust manifold was cracked along one of the welds. This was replaced.

A period of time prior to this P0420 issue, the idle had gotten rough. When the Vue comes out of auto stop while sitting at a light, the idle is quite rough. Once the P0420 code appeared, I presumed whatever was causing the code was causing the idle issue.

After a couple thousand miles from having the exhaust manifold replaced, the code returned. That is the only code that is set. The rough idle when coming out of auto stop never went away.

Any ideas or suggestions? I'm beginning to think that the cat is really bad and that the exhaust leak was around long enough to cause enough extra fuel to be dumped into the fuel stream to cook the cat.

Thanks for your assistance ...

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