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michael 1970 08-29-2012 04:08 PM

2006 Grand Cherokee code 0420
Hi All-
This past weekend I thought I would do a nice thing and clean and Detail my wifes car. I cleaned the engine as I have done numerous times before with Simple Green, taking care not to get the electrics wet.
Well the next day it started bucking, hesitating and tripped the MIL. Scanned it, found a random misfire. Pulled the coils, found moisture on 4 of them. Blew them off, sprayed with WD40, and blew off again. Blew out the wells where the plugs are, found a lot of water in # 2. put back together, still had a slight skip, cleared codes, reset the MIL.
Wife drove it the next day, a lot of hesitation and the mil came back on. Scanned and found # 5 cyl misfire. Pulled all coils, #5 had a little moisture on it, the rest dry. Replaced all plugs, blew out all wells again, reinstalled, reset MIL, took for a long ride, ran great!! nice and smooth.
Wife drove it again, MIL came back on, rescanned, got a code 0420- Catalyst System Low Efficiency
I have read that a faulty coil could cause this.
Does anyone have any Ideas?
Thank You.

2005JGC 08-29-2012 05:56 PM

I would just drive it for a while. Run it up and down the road at 3-4k rpms to get the cats nice and hot then just drive it normally for a week or so.

When you have a misfire it pumps raw fuel into the cat which just melts the cat down...

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michael 1970 08-29-2012 07:01 PM

Re: 2006 Grand Cherokee code 0420
Thank you, you said it melts the cat down, is that as bad as it sounds? are the cats that delicate?

2005JGC 08-30-2012 12:56 AM


Originally Posted by michael 1970 (Post 676859)
Thank you, you said it melts the cat down, is that as bad as it sounds? are the cats that delicate?

I'm not talking like make some toast warm, it gets HOT without the raw fuel... Typically they can take a bit of misfire abuse, but that is the reason the make the MIL flash. Because it cannot take a lot.

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michael 1970 09-03-2012 11:14 AM

Re: 2006 Grand Cherokee code 0420
Hi guys-
Well, I think that the cat is gone. I had a friend of mine who is a toyota tech come over and scan and diag. the code, and it appears the o2 sensors are working fine, and the misfire did the cat in. The car has 116K miles on it so the cat was prob weak before, due to the # of miles, and the misfire did it in.
Does anyone know of a place I can get a new cat? I assume jeep only sells the whole front pipe w/ the 2 cats.
I am just looking for options and cost.

2005JGC 09-03-2012 12:03 PM

Re: 2006 Grand Cherokee code 0420
honestly your safest bet is the rediculously expensive Y pipe from Mopar, I have seen a number of people who replace the cat with an aftermarket then continue getting catalyst effiency codes. There may be some users on here that have had a cat replaced with good luck and maybe they can tell you what brand they used, but ultimately your going to need to call up a local exhaust shop and they will be the ones who cut out a cat and weld in a replacement.

michael 1970 09-03-2012 06:28 PM

Re: 2006 Grand Cherokee code 0420
That was what I was thinking also. My tech buddy said he has replaced some cats w/ aftermarket units, and has had some troubles w/ reoccurring codes after a couple years. He says to try to go w/ the factory units, but if $ is an issue, then get an aftermarket unit w/ a lifetime warranty, so if there is any problems down the road, at least the part will be covered. He has returned some units for replacements.
Hopefully some other members here have gone thru the same issue and can give some advice regarding aftermarket units.

michael 1970 09-05-2012 08:15 PM

Re: 2006 Grand Cherokee code 0420
Well. I was able to find the OEM Y pipe from a online dealer, which is actually in my area, for $899, and no need to pay the ridiculous shipping charge for it. My next question is: do I need to remove the transmission crossmember to replace it, or is there enough room to drop the front of the pipe and slide it out before the rear of the pipe pivots up and hits the underside of the floor.

2005JGC 09-06-2012 12:55 AM

The cross member must come out. Just fyi, the collector hardware can SUCK... Make sure to get some penetrating lube on them early and ALWAYS tighten them some before loosening them!

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michael 1970 09-11-2012 06:11 PM

Re: 2006 Grand Cherokee code 0420
I should prob replace the o2 sensors while I have it apart. Should I go OEM, or aftermarket?

Frango100 09-12-2012 12:41 AM

Re: 2006 Grand Cherokee code 0420
Jeeps normally donīt like Bosch O2 sensors, because they heat up too slowly and give related fault codes.
OEM is best, which is in fact Denso.

michael 1970 10-26-2012 05:23 PM

Re: 2006 Grand Cherokee code 0420
Hello again-
I have not had the opportunity to replace the cats yet- trying to save some money. But I did have something odd happen regarding this issue. Over the last couple weeks, the check engine light would go off on occasion. It would go off, stay off for 1 to 2 days, then come back on. I scanned it for codes when it was off and the p0420 was still there, however there was not a pending code. When the light is on, there are 2 of the same codes, one of them being "pending"
Is this normal? I thought that the check engine light would come on, or go off after 3 driving events of either a problem or no problem.
I was thinking about changing the o2 sensors on the bank that has the issue to see what happens.
I thought that a cat either works or not. It seems that it is working from time to time (thus turning off the CEL) or not(turning it back on).
What do you think??

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