There are two primary reasons for cat failure; rich running or an ignition miss. Lack of maintenance is usually the cause, you should really be changing your spark plugs about every 40,000 miles even though they're platinum and the dealer recommendation is probably over 100,000. This is one of the worst mistakes manufacturers made in the last ten years, trying to market maintenance free vehicles. Spark plugs degrade and cause more unburnt gases to be released into the cat where it causes damage.
The other big one is the O2 sensor, these also degrade over time and should be replaced if they're signal starts to degrade. Only an experienced tech with a scope can measure this though. It's a good idea just to replace your primary O2's every 100.000 miles. Another cause could be an engine that burns oil, if excessive amounts enter the cat it will degrade rapidly.
Hope this helps.
As far as the dealer goes, somethings require them, tune ups should not if you have trust in a good local mechanic.
The engine knock question is a tough one. Do you mean knocking as in bad gas, or a continuous, "something must be broken in there" type of metallic banging?
$1200 for a tune up is ridiculous, especially if you just had it done by a reputable mechanic. I would get an estimate from the stealer...uh, I mean dealer and take it back to the 1st mechanic and say what's up? He can give you warranty on anything he's done if it is faulty. My supplier doesn't even blink If I return bad parts, but you must be reasonable. If this new estimate contains a front brake job, new O2 sensors, or coil packs that he never changed, then obviously that is beyond his control. Throwing away work that was done 5 months ago is just wasteful and stupid. Sometimes dealers think the sun shines out their rear and they have to do everything over again, it's not true. I actually end up doing a lot of diagnosis in my area for the local dealers.
Still too much. What is the mileage and engine size of the vehicle?
Why do they want to replace it again, are they saying it's plugged or is this an emission test or engine light issue? Anyway, this is what I'm guessing and suggesting: They are saying that you need the cat that is inside the exhaust manifold and the one under the car. Why? Not so sure they know, but I'm pretty darn sure they are not both bad if they are only a year+ old. I suspect you have a code recorded, probably a p042X, that says you need one or the other and they just want to shot gun both on there. A cat code can be caused by other things as well, but I'll assume they're right.
There are a couple options, you can get the rear cat online for about $275., the front one, about $300 off ebay You have to take the exhaust manifold apart to change it. Probably a couple hours for your mechanic to do both, that's $800. total So that's not too far off what the dealer said with their higher costs and the tune up. That's assuming you do need both. Here's what I would do, it sounds a little scary but would actually work far better, last way longer and be way cheaper. I would replace both those cats with one. You can buy what is known as a "super cat" it performs the function of both that are on your car now. These are not the cheap generic cats that they sell over the counter, this is something you have to ask specifically for. They are sold with or without an air tap in generally 3 sizes. They cost about $250. and can be welded in directly where your rear cat is now. The problem is, the front cat has to be taken out of the manifold or the new rear cat won't light off, and you may have to relocate an O2 sensor. There would be a couple hours labor involved. Total, less than $500, much better system and more servicable in the future. You have to find someone willing to do it though. It depends where you live, in my area, we have been dealing with emission repairs for 20 years so this repair is common.
If you can, give me some info, codes, reasons etc, for the cat replacement. Maybe you really only need one....or none.
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I just had a tune-up 5 mos. ago - new plugs, wires, etc. As to the engine knock - sorry - what I meant to say is the dealer said the engine was mis-firing (that is what dealer said) which caused gas to go directly into the cat - which caused the cat's (it has 2) to go again.
Also, what would you suggest i do about the fact that I just had a tune-up done at a reputable mechanic 5 months ago - who said he replaced wires, plugs, everything. The dealer says that I need a tune-up - that is what was causing the engine mis-fire which caused the cats to go. I am confused - they want another $1,200 to do it. I told them I want them to keep all the parts
The $1,200 was not just a tune-up - it is to also replace the 2 catalytic converters on the 2002 Santa Fe
106,000 miles - 2.4L 4 cylinder
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