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Unread 06-30-2010, 05:57 PM   #1
BillyEd
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What could foul both front catalytic converters?

Just my luck. I went to get smog checked, and during the test my check engine light came on, causing me to fail, in spite of having clean emissions.
The code generated was P0420, indicating problems with either the rear O2 sensor or the catalytic converter.
I took my '02 Cherokee to my mechanic and he says that both front cats are bad and need replacing for $600. (honestly, I didn't know I had more than one, but he tells me that there are three: two in front, and one in back).
What can cause both front cats to go simultaneously? The jeep runs perfectly, and I get 21 mpg on flat highways, and my emissions were really clean. I currently have 164,000 miles on it. I don't seem to be losing any significant engine oil.
I always buy 89 octane gas, but I buy it from whatever station sells it cheapest. Half the time it'll be a big brandname, and otherwise it will be an independent.
One time I added a bottle of Risalone oil treatment about 20k miles ago.
About every other oil change I add a bottle of gas treatment to the tank to clean the fuel injectors.
Last year the engine computer went out, and before I replaced the thing, it would run a bit rough, and sometimes blow blue smoke out the exhaust if I accelerated hard.
Does anyone see any red flags in what I've mentioned that would cause both cats to go bad?

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Unread 06-30-2010, 06:39 PM   #2
rm2001wj
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If that is the only code, then only one of the front cats is bad. It's always a good idea to include engine info on your Jeep (I assume Cherokee = Grand Cherokee?). For a V8 (4.7L), it's the cat on the driver's side. For an I6 (4.0L), it's the cat at the front (closest to the radiator).

It's not really a case of two cats going bad at the same time. Probably the mechanic realizing that if one is bad, then the other cat could go bad soon, just wants to replace both now.

If both were bad you should get a P0432 along with the P0420.

If you really meant "Cherokee" then you have posted in the wrong forum for your Jeep, but this info is still valid.
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Unread 06-30-2010, 06:41 PM   #3
Bane7962
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Quote:
Originally Posted by rm2001wj View Post
It's always a good idea to include engine info on your Jeep (I assume Cherokee = Grand Cherokee?).
X2 man. I got really confused for a second
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Unread 06-30-2010, 10:38 PM   #4
ras2001wj
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well, XJ's were discontinued in 2001, so I'm gunna have to say WJ
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Unread 07-01-2010, 05:50 AM   #5
rm2001wj
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I guess because he is a newbie he needs to get a few things sorted out. Profile says 2002 XJ Cherokee with a 4.8 engine.
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Unread 07-01-2010, 06:14 AM   #6
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oh boy do we have our work cut out for us setting him straight
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Unread 07-01-2010, 07:16 AM   #7
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I'd say the cat was bad just due to the mileage. Burning oil will kill them quickly also. And if you haven't replaced the o2 sensors, now would be a good time. Some say only use mopar sensors. I wouldn't worry too much about what fuel you're putting in, supposedly the only difference is what cleaners are put in. I don't know how true that is, my pickup never ran well on flying j or maverick fuel. Anyway ...

Contact Kolak, kolak@aol.com or pm kolak and see if he sells the whole exhaust system from manifold to tailpipe. I don't remember if he does or not. If it were my jeep I'd be working on replacing the whole system.
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Unread 07-01-2010, 02:37 PM   #8
BooBoo96ZJ
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Running rich will kill the cat the fastest. Burning coolant will kill oxygen sensors..Fast.
If you have a oxygen sensor thats constantly reading lean. It will trash your cat. The more unburnt gases you have flowing around that faster your cat heats up. the hotter it gets the faster it goes out.
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Unread 07-07-2010, 02:44 PM   #9
BillyEd
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Quote:
Originally Posted by rm2001wj View Post
If that is the only code, then only one of the front cats is bad. It's always a good idea to include engine info on your Jeep (I assume Cherokee = Grand Cherokee?). For a V8 (4.7L), it's the cat on the driver's side. For an I6 (4.0L), it's the cat at the front (closest to the radiator).

It's not really a case of two cats going bad at the same time. Probably the mechanic realizing that if one is bad, then the other cat could go bad soon, just wants to replace both now.

If both were bad you should get a P0432 along with the P0420.

If you really meant "Cherokee" then you have posted in the wrong forum for your Jeep, but this info is still valid.
Sorry for the late reply - I've been on the road and offline for a bit.
It's a 2002 Jeep Grand Cherokee Special Edition with a 4.7L engine (4.8L in my profile was a typo error, which I just fixed).
I was talking to a mechanic recently, and he thought that the prior bad engine computer was the likely culprit for fouling the cat converters.
I diagnosed the bad engine computer by seeing multiple codes for bad fuel injectors. Every time I checked and re-checked the codes, it mentioned different fuel injectors were bad - one time it would say #'s 1 & 8 were bad, the next time it would say #'s 4, 5 & 6 were bad, etc. The exhaust smelled very rich, as well. It ran only slightly rough, but blew smoke when hitting the accelerator to pass. Nothing like throwing out a smoke screen when passing someone! I ordered a remanufactured engine computer from All Computer Resources Dodge Jeep Chrysler Ford GM OEM Engine Computer Auto Computer Dodge PCM Chrysler PCM Jeep PCM Store and popped it in - problem solved, but that was after running it for a couple of weeks like that.
At any rate, the mechanic I spoke with thought that running it overly rich when the computer was bad would damage both front converters equally badly.
I don't think any significant oil is running through the exhaust, as the oil level remains constant between oil changes.
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Unread 04-19-2013, 10:35 AM   #10
sachin29
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Grand Cherokee 2003 Front Cats fouled

I had the exact same issue when I bought a used 03 WJ Grand Cherokee, 4.0 Engine.
The cause was simply overheating of engine and since the front CATs are closest to the Exhaust Manifold, the Cat contents burn first and you get a rotten egg smell inside the car.
Also the Rad Fan Relay was bad leading to the overheating. I went thru 3 exploding Radiators before I realized the cause.
To pass emissions I drilled out the content of the Front Cats as the assembly was about 700$ to install. I then installed a good Walker rear Cat converter and replaced Rad Fan relay and used a Rad flush( I drove around for 3 days and then drained out the coolant). Replaced the coolant and ALL Problems were solved.
Since I still get get the same code and the mileage is bad, I am now customizing the front Cats with cut & paste with part #
Part No 93270 from Walker Cat converters purchased from Advance Autoparts.com.
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