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  #1  
Old 08-06-2008, 02:39 PM
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Replacing the Catalytic Converter

Hello,
I have a 98 3.0 2WD and driving the other day the CEL came up with a P0420 code. The Ford dealer ran the diagnostic test and said the cat had to be replaced at a cost of 1400.00. To his credit the service rep called around and got a better price of 800.00 at a local muffler shop. I called NAPA and got a quote of 490.00 for the cat. Is replacing the cat a simple job that one or two person(s) can do. Looking at the tech manual it seems pretty easy to do with the exception of the rusted bolts. I was planning on asking my neighbor if he could help me replace it.
Thanks,
JM
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  #2  
Old 08-06-2008, 02:44 PM
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Careful, aftermarket cats a lot of times have to be welded in. The old one is cut out and the new one welded back in place. If you have the equipment you can do it yourself. Just check first.
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Old 08-06-2008, 02:50 PM
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Old cats don't die. They are poisoned. Something has coated the catalyst or has killed it. You should be wondering what did it in. My old truck could buy beer, and it has the original. My old car has enough mile to go to the moon, with the original cat still catting.
I don't know 420, but if it is indicating poor cat performance, it is done using a second O2 sensor after the catalytic. Sometimes it is the O2 sensor that is failed, instead of the catalytic, indicating poor performance where there is actually no problem.
Why did it die, if indeed it is dead? NAPA should be able to provide a bolt-in replacement, or you can possibly get a generic from JCWhitney for about half that (well, you could a while ago)
tom
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Old 08-06-2008, 02:53 PM
ranger88a ranger88a is offline
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The code indicates a weak cat not an o2! and cats do get weak! Code for o2 would be slow responce or lack of switching codes. Po420 is a cat code because the cat isn't doing it's job.
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Old 08-06-2008, 03:18 PM
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Trust me I did as much research on this CEL code as possible. I even asked if there was any possible way the o2's could have caused this problem. There seems to be much discussion on this topic and I am hoping that I am not throwing my money away on a not needed part. Lets play devils advocate on this, if the cat is replaced and the code comes back, do I have any recourse with Ford because they did an improper diagnosis.
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Old 08-06-2008, 04:34 PM
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If they do the work you do!
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Old 08-06-2008, 04:53 PM
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If it were mine and I had to change out the cat, then I would also change out the after-the-cat 02 sensor, simply as a matter of course. That's just how I am. And because of that, before actually replacing the cat, I would first replace the aforementioned 02 sensor and see what happens. In my case, there would be nothing to lose by doing that.
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Old 08-06-2008, 05:27 PM
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Thanks, this may sound funny but before I took it to Ford the CEL went out and has not come back on. I may have jinxed myself but now what, spend the money on a new cat, new downstream o2 sensor or wait it out.
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Old 08-06-2008, 08:35 PM
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Hi Flow cats run 150 installed around here. No need to put OEM cats back in.

Any Mustang/Corvette etc change out to hi flows when upgrading exhaust almost always.
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Old 08-10-2008, 03:17 AM
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10 times out of 10, a bad cat code means there is a bad cat- NOT bad 02s.

So called universal cats are very inexpensive and work just as well, but fit up is often terrible. I threw one in my dad's taurus a few months ago and I spent hours heating/ bending/ welding pipes to get it to go. Even an oem style, as ranger88a mentioned, often takes some welding and bending to get it right.

As with most things, cats do not last forever. Yea, they can be killed off prematurely by contamination of some sort, but even the best tuned engine with mega hours on it will eventually stop a cat from working properly.
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Old 08-10-2008, 05:01 AM
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If you want to find out why Cats are so expensive, call your local junk dealer and ask what he is paying for used ones. There is some kind of valuable components in a Cat and as you are probably aware, the price of junk scrap metal is soaring. The thieves are even removing them from vehicles parked in lots. SUV's are a prime target because then don't have to jack them up. They use a cordless sawzall and have it out in a matter of seconds.
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Old 08-10-2008, 11:32 AM
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Quote:
Originally Posted by seagull369 View Post
10 times out of 10, a bad cat code means there is a bad cat- NOT bad 02s.

So called universal cats are very inexpensive and work just as well, but fit up is often terrible. I threw one in my dad's taurus a few months ago and I spent hours heating/ bending/ welding pipes to get it to go. Even an oem style, as ranger88a mentioned, often takes some welding and bending to get it right.

As with most things, cats do not last forever. Yea, they can be killed off prematurely by contamination of some sort, but even the best tuned engine with mega hours on it will eventually stop a cat from working properly.

The direct fits we sell ALWAYS fit perfectly without any alterations. If you had one that required modifications, you bought it from the wrong place.

The second statement is false. The cats can and sometimes do last forever. Perfect tuning a good quality fuel is all that is required. If the engine starts to burn oil, then that obviously isn't perfect tuning. My dad has a 1996 Ranger with well over 300,000 miles, and it has the original cat still functioning like new. Our 2000 Dodge Dakota has 280,000, and its not running perfectly, has a oil burning problem. Regardless, everything else is kept tuned and it still has the original.

Let me do a brief rundown of how catalyst monitoring works so you know what you really need to replace, and how to really isolate what caused the failure.

The vehicle used the primary O2 sensor(s) for two main purposes. Firstly to make minor adjustments to the fuel trim. Secondly to make sure the oxygen levels in the exhaust are always changing. Modern catalysts use a compound called ceria, which contains cerium, as a means of storing oxygen. The reason this is required is because a 3 way monolith catalyst cannot breakdown NOX and HC at the same time. NOX is best reduced in an environment that is low in oxygen, otherwise the free nitrogen would simply recombine with oxygen and form more NOX. Likewise, HC needs oxygen to be converted to water and carbon dioxide. However it was discovered that one could satisfy both requirements by storing the oxygen. This allows both functions to function efficiently all of the time, but in order to work, the oxygen levels must constantly change. Otherwise the oxygen storage would either completely saturate, or all of the stored oxygen would be used up and the efficiency overall would suffer.

The rear O2 sensor monitors the catalysts efficiency by monitoring the oxygen levels after the cat. If the catalyst is working properly two things will occur and can be observed on a graph. One is that the oxygen levels after the cat are significantly lower than the levels entering the cat. The second is that the signal of the rear O2 is fairly stable, and does not oscillate as strongly as the front O2 sensor. When a cat has failed, the rear O2 signal will mirror the front sensor.

Catalysts are generally damaged by one of several things things. Either by exposing them excessive fuel, oil, or antifreeze, by overheating them, or by chemically contaminating them with either acetic acid or sulphur. When this happens, the precious metal loadings are rendered ineffective and the cerium is damaged or chemically altered.

The rear O2 sensors do not need to be replaced and here is why. If the sensor gets old and worn, its signal strength drops and its response time decreases. The behavior closely matches the behavior of a good catalyst. So long as the rear O2 sensor does not trigger a code for its own failure, there is no reason to replace it.

The front O2 sensor(s) on the other hand are a different story. If they go bad, their signal strength starts to drop, which indicates a lean condition, triggering the computer to add more fuel. The response time may deteriorate somewhat, but the sensor will damage a converter LONG before it sets a code for a sensor failure. For this reason, I recommend replacing the front O2 sensor every 80,000 - 100,000 miles, and also replacing it whenever you replace a catalytic converter. I would say as a general rule, 3 times out of 5, a failed converter is due to a failed upstream O2 sensor.

We have the cat you need for your job, and we have a lot cheaper than Napa, and we can guarantee it will fit perfectly. I don't know what part your Napa carries, but most of the brands that parts chains carry are not from companies that care about fit, and their catalogs are frequently incorrect. The product lines they choose to carry are dictated mostly by price, which will most of the time cause them to buy brands that are not test fit, and are not made to OE specs. We actually supply a few local Napas, there is no national supplier for Napa on catalytic converters, so they usually get them from a regional distributor.

We do not have the part for your application listed on our site, but we do have it in our inventory, which is extensive. If you are interested, just visit our website and use our direct fit parts request for to tell us what your application is, and one of our reps will get back with you with pricing and ship time.
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  #13  
Old 08-10-2008, 04:48 PM
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I appreciate the responses but since the CEL went out and so far I have not noticed any drop in performance I am going to hold off replacing the cat. I had a local muffler shop take a look the cat and as far as they could tell, the cat was okay and did not recommend replacing it. They did recommend replacing the o2 sensor first if the CEL came back. How hard is it to replace the upstream o2 sensor on a 3.0 engine.
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Old 08-11-2008, 07:09 PM
Bear River Bear River is offline
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Not too hard, but spray it will a good penetrating fluid and let it soak a couple hours. You should also use the correct wrench so you don't round it off. If you leave them in too long, they get harder to remove.
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Old 08-11-2008, 08:09 PM
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I have seen cats go bad for no reason whatsoever. Some cars are especially bad for this (Honda's especially)... typically they do last a long, long time, but it is not uncommon for a cat to become chemically ineffective with no internal damage or deterioration being present.

Ranger cats DO last a very, very long time and it's extremely rare we have to replace these on them (and the ones we do are usually contaminated with oil, antifreeze, or were physically damaged). If anything else the outer shell will rust apart before the internals go bad.

The internals are made of some precious metals, including Platinum, which is one reason they are so expensive. "Direct Fit" converters bolt right on, typically with no modifications being necessary. "Universal" converters usually have to be welded in place. Some cars are very touchy as to having the correct converter installed, typically vehicles with California Emissions or LEV or SULEV rated vehicles. For those I usually recommend the 'proper' converter assembly from the manufacturer.. but these (as you already know) are very expensive. Why? Because they are built and calibrated for that specific vehicle.

Rangers typically aren't so touchy, so a OBD-II compliant "Universal" converter should do the trick.

Your P0420 code can be caused by an exhaust leak, converter contamination, a misfire condition (or rich-running condition), or in extremely rare circumstances... oxygen sensor(s).

Here's how it works. O2 sensors measure the oxygen content in the exhaust. The front sensor should constant have variable voltage... between .1 and 1.0 volt. Anything below .5 volts is considered lean and anything above is rich. If you were to graph the front O2 readings, it would look like a sine wave. It gets rich.. the computer leans out the fuel mixture.. it gets lean.. the computer enrichens the fuel mixtures... many times a second.

The rear O2 sensor is a different story.. it's reading should not switch very much. Why? Because the converter stores oxygen. As the exhaust passes through it, it basically burns very hot, mixing in the the stored oxygen when needed. When the exhaust is unloaded, the excess oxygen from the engine once again gets stored into the converter. Over time these do lose their ability to store oxygen, and as this happens the rear O2 sensor begins to switch. When it switches with a frequency becoming similar to the front O2 sensor, this is when the P0420 sets.

Now, when you have incomplete combustion (from a rich condition or a misfire) you get a lot of extra oxygen passing through the exhaust. Once the converter cannot hold any more oxygen, it has to let the rest pass through the rear O2 sensor, and it begins to switch. Likewise, an exhaust leak in front of the converter can allow extra oxygen to enter and cause a similar outcome.

In the old days there was also the possible of contamination from gasket sealant, but the vast majority of sealant manufacturers have changed their formulas so this is no longer the case.

It's possible your engine had a 'hiccup'.. .maybe a little misfiring due to whatever reason.. not enough for the computer to pick up on, but enough to fool the oxygen sensors. It's also possible your converter is in borderline condition... right on the line between pass and fail.

In either case, P0420 isn't going to cause any driveability problems (unless the converter is restricted due to contamination). I wouldn't worry too much about it unless the light comes back on, or unless you do experience any other driveability symptoms.

This article in Wikipedia explains everything pretty well: Catalytic converter - Wikipedia, the free encyclopedia
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Old 08-11-2008, 08:09 PM
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