Error Code P420 and timing chain replacement? - GMC Acadia Forum: AcadiaForum.net
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post #1 of 14 (permalink) Old 05-04-2016, 08:38 AM Thread Starter
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Error Code P420 and timing chain replacement?

My check engine light recently came on. When I hooked my code reader up to it, I received the P420 (Catalyst efficiency below threshold Bank 1). I immediately thought it must be a problem with the catalytic converter. However, my scan tool gives suggestions for the most common fixes based on vehicle year, make, and model. and it came back as "Replace Timing Chain"! huh?? Does anyone know if a stretched timing chain would cause that P420 code to kick out?? Thanks!
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post #2 of 14 (permalink) Old 05-04-2016, 09:17 AM
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Re: Error Code P420 and timing chain replacement?

That's one of the possibilities - there are many. Best to go to a professional service center if you can't figure it out on your own. It would be good to know what year, model, and mileage you have.

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post #3 of 14 (permalink) Old 05-04-2016, 09:40 AM Thread Starter
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Re: Error Code P420 and timing chain replacement?

Sorry, you're right about knowing the vehicle info. I have a 2009 Acadia with approx 92K miles.
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post #4 of 14 (permalink) Old 05-04-2016, 10:39 AM
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Re: Error Code P420 and timing chain replacement?

If the vehicle has a stretched timing chain issue, it can be repaired under SCA #11340B - Timing Chain Wear. This Agreement (essentially a warranty extension concerning a specific item) is good for 10 years/120,000 miles, whichever comes first, regardless of ownership. Look at the 'Recall' thread (http://www.acadiaforum.net/index.php?topic=10487.0) and others concerning stretched timing chains for details.

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post #5 of 14 (permalink) Old 05-04-2016, 10:46 AM Thread Starter
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Re: Error Code P420 and timing chain replacement?

Thank you. I will look into that. I am still confused as to how a timing chain issue would kick out the P0420 error code though!
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post #6 of 14 (permalink) Old 05-04-2016, 11:23 AM
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Re: Error Code P420 and timing chain replacement?

In a nutshell ... Valve timing is one of the things that effects the way fuel is burned. Catalytic converters don't really like unburned fuel passing through them. After a while, they become inefficient at handling the unburned fuel. What surprises me is that there aren't any other fault codes present.

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post #7 of 14 (permalink) Old 05-04-2016, 01:37 PM Thread Starter
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Re: Error Code P420 and timing chain replacement?

The absence of other fault codes surprised me too after doing a bit of online research. I will just take it over to the dealer and see what they come back with.
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post #8 of 14 (permalink) Old 05-04-2016, 03:48 PM
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Re: Error Code P420 and timing chain replacement?

FWIW, I saw a similar situation with my 2010 at 125,000 miles. P0420 occurred with timing chain code P0008 (Bank 1 Engine Performance Inefficiency). P0420 was eliminated by replacing all six coil packs. After 6+ years of continuous use, there was no point in replacing less than six. Coil packs performance can deteriorate, like everything else. Inefficient coil packs will also contribute to and magnify poor combustion. In my case, at least with new coil packs, the unstable idle that accompanied the P0008 was improved. Coil pack fix - relatively cheap DIY; timing chain replacement - painfully expensive. So yes, P0420 and P0008 can occur at the same time. Good luck!

'10 Acadia SLT1 AWD Med Brown Metallic 230,000+km
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post #9 of 14 (permalink) Old 05-05-2016, 04:53 PM
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Re: Error Code P420 and timing chain replacement?

I, too, just had my timing chain and catalytic converter replaced due to codes P0008 and P0420 about a month ago. My 2010 had about 93,000 miles on it.

I hope I didn't just pay to have the CAT replaced for nothing and the real issue is the coil packs as was the problem with Craig's.

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post #10 of 14 (permalink) Old 05-05-2016, 08:32 PM
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Re: Error Code P420 and timing chain replacement?

Kingsfr - As part of diagnosing the P0420, I looked at the output from the O2 sensor on the Bank 1 CAT using my Flukemeter (has a basic scope function). Clearly, the upstream sensor was performing less than ideal, so it would have been easy to conclude that the CAT was faulty. However, my approach was to rule out poor combustion first. Also, a new GM CAT costs a small fortune, so no need to jump to conclusions too early. Plugs were due for replacement, so it was a good place to start - didn't rectify the P0420, but it did run better. When the coil packs were replaced, the idle settled down considerably and the P0420 code vanished. Re-checked the output and waveform from the Bank 1 O2 sensor and it was perfect. I'm certain that a few CATs have likely been prematurely replaced due to shortcuts in troubleshooting. In my case, it was unneccessary. The scope offered irrefutable evidence that this was the 'right' fix.

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