2003 Malibu codes 0420, and now 0303

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  #1  
Old 09-25-14, 12:30 PM
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2003 Malibu codes 0420, and now 0303

I have a 2003 Chevy Malibu, v6, 3.1, auto, 140k. Two years ago I had the catalytic converter replaced because it was clogged due to the very common intake manifold gasket leaking coolant (thanks GM) - this problem was fixed two years before. Cost $450. Now, some two years later I have been getting the exact same error code (P0420) saying catalytic efficiency below normal (or some such wording, which usually means a bad C/C). When I called the shop that installed the C/C, they told me that aftermarket C/C's only last 2-3 years (wow). Now, some 8 months of driving with the code 0420, I am also getting code 0303 (misfire in cylinder 3). Plugs and wires are about 5 years old, never replaced O2's. I can only now feel the car hesitate and be sluggish. So some questions - is it true that aftermarket C/C's only last 2 years? Is the new code (0303) just a coincidence or could that have caused the other problems (although code 0420 showed up 8 months before 0303)? I plan working from cheapest to most expensive - check spark from electronic ignition then plugs then wires then O2's then C/C but is there a better plan to address this? Thanks
 
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Old 09-25-14, 07:24 PM
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the 420 does not always mean the cats are bad, could just be gas filled(in the form of carbon) due the miss you have been having. the miss might have been very slight for the longest time. i would have the miss fixed and have a fuel system or upper intake clean done. this should clean out the cat. another theory is that you have not kept true to a gas station or go to cheep(quality) stations (walmart, costco, kroger, swifty, these dont always carry the same type of gas). this causes the computer to consistently adjust to the gas, making it not efficient and ether over fuels or under fuels the cat. at which over time makes them carbon up and if not taking care of they will clog. with both cases you still should start with fixing the miss and cleaning out the system.
 
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Old 09-26-14, 05:35 AM
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shinrich, have the spark plugs been changed?
If so, what new plugs were installed... original equipment AC plugs?
 
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Old 09-26-14, 06:05 AM
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update

Thansk for the replies. Here is an update. Coming to work today the check engine light started to flash and although I was still going pretty fast up a hill, I felt a loss of power and it wouldn't kick into overdrive. Later the light stopped flashing. checked codes again and have four codes - two 0420's and two 0303. (Why did it duplicate the codes?). Thanks
 
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Old 09-26-14, 10:08 AM
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You didn't answer my question yet ?? .......
 
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Old 09-26-14, 10:36 AM
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Plugs and wires were changed about 5 years ago when I fixed the intake manifold gasket leak. Thanks.
 
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Old 09-26-14, 11:10 AM
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Here is an update. Coming to work today the check engine light started to flash
Ok, do not continue to drive your car with a flashing CE light condition. Doing so can, and often will, result in further cat failures.

And yes, GM has a horrible record of inexcusable intake manifold leaks (in part due to the company's lousy coolant). This GM coolant should always be completely and thoroughly flushed out and replaced with a quality aftermarket coolant in order to avoid further problems.
 
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Old 09-26-14, 11:39 AM
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Had a very similar situation with my 3.4 monte carlo. Had a shop replace the intake manifold gaskets and cat but they did a p*ss poor job. Wound up with leaking gaskets and P0420 code barely 10K miles later. This time I redid the repair myself and also replaced the plugs (ACDdelco platinum) and wires (BWD from Oreilly) but still had the P0420 code.
I found out the 2.8/3.1/3.4 cars are overly sensitive with their converter readings and aftermarket converters frequently trip the code (plus the contamination from the leaky gaskets). I had seen multiple postings from people claiming Thunderbolt converters did not have this problem but I was skeptical because I could get one for under $80 shipped. I also saw claims that said a good dose of seafoam in the gas along with getting the converter really hot would *sometimes* get it clean enough to clear the code.
So...I put a whole 16 ounce can of seafoam into my tank and topped it off with premium gas. I then cleared my code with a scanner and took the car on 6 separate 10 mile freeway runs with the tranny in low2 gear so at highway speed I was running about 3400 rpm's or so. Checked for codes again and I had the P0420 in 'pending' status but it hadn't tripped the light yet. I drove the car normally after this and the light never came back on. When I scanned for codes again the 'pending' status had cleared. It's been about 3 months now and the light was always on before. I'm sure this won't work for everyone but in my case luckily it did.

One more note if you replace the O2 sensors, don't get Bosch, these engines don't seem to like them. I would try Denso since that is who actually makes ACDelco O2's to save the markup for having the ACDelco name on them.
Good luck, these engines definitely have their quirks but when fixed correctly can run a long time.
 
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Old 09-29-14, 05:43 AM
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More Follow up

I replaced the plugs and wires this weekend. Still getting the 0303 code (misfire on plug 3). I noticed that normal driving seems to be ok, but once I get it out on the highway and try to accerate, that is when I feel the loss of power, also the flashing check engine light came on again - also when I was out on the highway, but went away once I slowed down and got off the highway. Since I keep getting the misfire on plug 3 (which would explain the lack of power and the code 0303), what else could it be - ignition module? Internal cylinder problem? What is my next step?
 
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Old 09-29-14, 07:55 AM
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Another question, since this seems to be only cylinder #3, does that mean it is could be a fuel injector? If an ignition module went bad, would that only effect one cylinder or all of them? I've never had an injector go bad so am unfamiliar with what happens if they do go bad. Would that give me the code 0303? The heat shield on the cat is loose and the alt pully squeals so I've got A LOT of noises coming from the car, but I believe I can also hear an engine tapping that was not there before. Could that be caused by a bad injector or bad ignition module or would that indicate internal cylinder issues - bad valve maybe? I am trying Sea Foam and B-12 to see if cleans up a clogged injector (and because they were cheap and I didn't know what next to do). Any suggestions?
 
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Old 09-29-14, 12:35 PM
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How about this - can I switch out the ignition coils - putting the currently 6/3 coil in another position on the car and see if that changes where the miss-fire is? I know the coils are labled, but aren't they really interchangable? How hard is it to swap out the coils?
 
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Old 09-29-14, 12:39 PM
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I'd follow mnJim's advice of not going with Bosch. I've had nothing but bad luck with anything Bosch going back to '71.

Anyway, First double/triple check the simple stuff like firing order and vacuum leaks (especially around the oil filler cap and int. man. areas). Then I'd check the compression and fuel pressure (low F pressure will usually cause a random lean miss fire rather than just one cylinder tho). Another thing to check would be exhaust manifold leaks (especially at the heads).
 
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