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Discussion Starter · #1 ·
Hey all,

I have a 2002 ZX5. I've had the check engine light for a while and want to finally take care of it. The code is P0420. I know this is related to the catalytic converter and O2 sensor(s). I understand, for the most part, how the system works (that the efficiency of the cat is measured by the difference between the upstream and downstream O2 sensors). I have replaced the downstream O2 sensor since that seems to be more common than the actual cat "going bad" and because it was cheaper. However, the light still comes on. It varies as to how long it take for the light to come on, but from what I hear, it takes a certain amount of city and highway driving in the same trip for the sensor to actually read so I don't think the time it takes for the light to come back on means much. I did have a valve cover gasket leak a few months back (and the light has been on since I replaced the gasket and I believe before that) and there was a bit of oil where it shouldn't be. From what I understand this could have caused my cat to go bad, right? Also, my car has about 130,000 miles and I know they can go bad after 150,000 anyway.

I guess my main questions are:

Does it seem very probable that I need a new catalytic converter because of the oil leak problem and also because I've replaced the downstream O2 sensor?

How come I don't see much about the possibility of the upstream sensor being bad? Is there a different code that sees when the O2 sensor isn't working at all? Or could I need a new upstream sensor too?

Thanks in advance for any help that you can provide!
 

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Hey all,

I have a 2002 ZX5. I've had the check engine light for a while and want to finally take care of it. The code is P0420. I know this is related to the catalytic converter and O2 sensor(s). I understand, for the most part, how the system works (that the efficiency of the cat is measured by the difference between the upstream and downstream O2 sensors). I have replaced the downstream O2 sensor since that seems to be more common than the actual cat "going bad" and because it was cheaper. However, the light still comes on. It varies as to how long it take for the light to come on, but from what I hear, it takes a certain amount of city and highway driving in the same trip for the sensor to actually read so I don't think the time it takes for the light to come back on means much. I did have a valve cover gasket leak a few months back (and the light has been on since I replaced the gasket and I believe before that) and there was a bit of oil where it shouldn't be. From what I understand this could have caused my cat to go bad, right? Also, my car has about 130,000 miles and I know they can go bad after 150,000 anyway.

I guess my main questions are:

Does it seem very probable that I need a new catalytic converter because of the oil leak problem and also because I've replaced the downstream O2 sensor?

How come I don't see much about the possibility of the upstream sensor being bad? Is there a different code that sees when the O2 sensor isn't working at all? Or could I need a new upstream sensor too?

Thanks in advance for any help that you can provide!
First off, yes there is a small possibility that the valvecover leak accelerated the failure of your catalyst. My reasoning here is that your PCV system is by definition a metered vacuum leak through the crankcase. If the valvecover gasket leaked bad enough it may have allowed the system to draw too much air through the crankcase and thus the engine would run lean until the PCM compensated. At idle the fuel tables would be a bit goofy but off idle they'd be normal. If the PCM really had a hard time keeping control of the fuel mixture as you move around the load tables it's possible that the cat had to work harder than it would otherwise. This is one hell of a stretch though. It is 1000 times more likely the cat is just used up from mileage, driving habits, fuel quality, etc. than a consequential failure from a bad valvecover gasket. Possible? Yes. Likely? No.

Second, this code means the two o2 signals are too similar. Basically, the PCM tries to drive the upstream rich 50% of the time and lean the other 50% of the time to maintain stoich. Because conventional 02 sensors are only good at detecting the presence or lack of oxygen - the PCM must play games with fuel delivery. Wideband sensors can actually quantify the oxygen content and therefore can infer an actual air/fuel ratio. Conventional sensors are sorta "dumb" in that regard so in order for the PCM to find the ideal air/fuel ratio it runs fat half the time and lean the other half. Average that over time and it ends up in the middle, which is basically stoich.

So.....if the PCM sees that this rich/lean flutcuation in the downstream, it knows that the catalyst is not storing the extra oxygen from these momentary "lean" events. This pretty much means the cat is no longer functioning effectively enough to control HC and CO emissions. The PCM only sets a P0420 code if the switchrate of the downstream sensor is 1.5 times the emissions standard. In other words, the code only sets when there is a significant lack of efficiency from the cat. This code cannot be set by a bad 02 sensor. This is a common misconception.


So.....your cat is almost certainly bad and likely needs to be replaced. You have a few options though:

Sometimes the material in the cat gets saturated with carbon and is no longer exposed to the compounds in the exhaust gasses. This can be due to driving habits, shitty fuel quality and poor fuel metering from other PCM inputs. If the car is running well with no fuel control issues (like vac leaks, MAF contaminationl, etc.) you may be able to "clean" the cat out and get a little extra life from it.

I would run it as low on fuel as you possibly can. Get fuel from the best reputed station in your area and if at all possible get something with very little ethanol content. Beat on your car like it owes you money. The trick here is to get a new type of fuel in it (in case fuel quality is contributing to your cats inability to store oxygen) and then try to "blow" out all the carbon that may be built up. Get the motor hot and do several WOT accels to redline. You may be lucky enough to extend it's life a bit.


Disconnecting the batt to clear the codes/tables can hurt you or help you in this scenario. It could oversaturate the cat in the process of relearning and negate all your efforts thus far OR it could correct whatever funky fueling strategy that's already there and perhaps that funky strategy is contributing to your effeciency codes. Hell, try it once and if it doesn't work disconnect the batt and whoop it's ass again. Worst-case the cat has to be replaced anyway.


If you DO decide to replace the cat I'd advise against an aftermarket unit. I don't have personal experience with the CFM unit but you get what you pay for when it comes to cats. The aftermarkets are cheaper because they are generally made of inferior quality. They often have a shorter lifespan.

G/L
 

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Discussion Starter · #3 ·
Grinder,

Thank you so much for the information. I guess I should've stopped here BEFORE I replaced the other O2 sensor. I will just replace the cat.

What do you mean by "I'd advise against an aftermarket unit"? Does that include anything other than one directly from a dealer? I had planned on getting a direct-fit one from a company out of Pennsylvania here. It looks like it would be a little over $200.

Any other suggestions anyone?
 

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Grinder,

Thank you so much for the information. I guess I should've stopped here BEFORE I replaced the other O2 sensor. I will just replace the cat.

What do you mean by "I'd advise against an aftermarket unit"? Does that include anything other than one directly from a dealer? I had planned on getting a direct-fit one from a company out of Pennsylvania here. It looks like it would be a little over $200.

Any other suggestions anyone?
A direct-fit unit means that it will bolt on and not require welding and should have nearly identical physical demensions of the original. Universal non-OEM catalysts often require that a shop fabricate a way to mount it to your exhaust pipe (they cut the old off and weld the new on).

Factory (Ford) replacement parts will likely be the highest build quality, easiest installed and longest living cat your money can buy. The precious materials in the cat that make it expensive cost the same regardless of who makes it. Thus, cheaper cats are often inferior build quality.....it doesn't mean Acme gets a better deal on the materials than Ford.
 

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Discussion Starter · #5 ·
Thanks again for the input. I understand the difference between the direct-fit and the universals. I was just wondering if you were highly recommending getting one from the factory only or if you were just recommending direct fit. It sounds like you were recommending from Ford.

I called 2 local Ford dealers and they are quoting me $1300-$1400 for the new catalytic converter. (That's just the part, I'll install it myself. I just wanted to clarify since the price is so outrageously high.) I'm going to be going with an aftermarket part. There's no way the factory converters are worth that much more!! ($1300 vs. $220)
 

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Hatch Nation #136
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The factory cat is a one piece manifold cat unit. That's why the dealer wants so much for it.

Watch out for aftermarket units that are cat only, because it WILL require welding.
 

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Discussion Starter · #7 ·
benkap,
Thanks for the input of the manifold and cat being one unit from the dealer. That makes a little more sense.

Are you certain that even the direct-fit aftermarket cats need to be welded?? Does anyone else have experience with this? I was pretty sure that the direct fit units would bolt on still. Anybody?
 

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Discussion Starter · #8 ·
Can anyone who has changed a cat on a 2002 Focus (2.0L DOHC) confirm or deny that the new cat has to be welded on, even with a direct-fit cat? I'll try to take a look at it this weekend, but there is a shield that blocks my being able to check quickly.
 

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Discussion Starter · #9 ·
Anybody? I would've thought someone would have changed a cat and could let me know.

Or even if benkap can clarify whether a direct-fit will still need welding if the manifold is not attached or not.

Thanks.
 

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Hmmmmm
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i ordered a magnaflow direct fit high flow cat and no welding required. straight bolt on
 

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Discussion Starter · #11 ·
drunkyoda,
Thank you so much for you input. I assume it didn't have the manifold attached as the one from for apparently does? Thanks again!
 

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Hmmmmm
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the one from ford does not. as you can order the manifold from ford also. they would not combine two things that bolt to eachother... that is just stupid. but i also agree with thinking that they wayyyyyy over charge for the cat. i would suggest getting the magnaflow high flow. its like 220 and makes a world of difference
 

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Discussion Starter · #13 ·
Thanks for the suggestion drunkyoda. I did a quick search and see that the high-flow cat from Magnaflow is about $220, as you said, and that's about what I was looking to pay for the other one that I found. But I like that it'd be from a trusted name like Magnaflow and that its high-flow. I'll certainly be considering that. Thanks!!
 

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Hmmmmm
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no prob. and trust me yo will notice a difference. i have read on the forum it is good for about 7-10 whp.

also throw some rep my way [thumb]
 
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