Thank-you. But unfortunately I am a little far from there.

Well, the spark plugs being worn out could have caused the misfires, it would have been pretty obvious to you that the engine was misfiring if it was bad enough to ruin a cat though. The info you gave is that you had a check engine light, and AFTER the plugs were replaced THEN the engine ran badly, not BEFORE, or did it run badly before replacing the plugs?
The problem as I see it isn't that you had a check engine light which they blamed on misfires and they replaced the plugs, what bothers me is that they are saying the cats are now ruined, but the truck only ran rough after they replaced the plugs.

Gilly

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No, it did not run badly before. Anyway, I went ahead and replaced both cats and all four oxygen sensors yesterday. Now, I am just keeping my fingers crossed. I hope this fixes everything. So far the CEL has not come back on!

Most oxygen sensors last about 60000 miles before needing replacement.
Replacing the sensors is a good thing ...... Not one but all four is best.
The sensors are part of the circuit that controls the fuel mixture. This could definitely foul a plug or cause a misfire due to fuel mixture. Too rich a mixture will cause a buildup of carbon in the cats and to lean a mixture will cause them to run hot and possibly burn the honeycomb inside. Not to mention fouled plugs.
Needless to say fuel mixture is critical. Does this Vehicle have plug wires? It may be time for a new set.
On some newer vehicles there is a coil pack (module) for each cylinder. In some cases one coil pack for two cylinders. I don’t know your vehicle. Have the hi-voltage ignition leads / coil packs checked. Some times you can see the ignition leads arcing at night. A sure sign that the insulators are dry rotted.
I recently replaced the oxygen sensor and complete exhaust system on my 1990 300SE. Including the cat. What a differance!
I did this after a tune up with plugs wires cap and rotor and still failing the emmissions test.....
When I put the car on the Dyno I found the power curve droping off over 3300 RPM. I cut the cat open and the honey comb was melted ....and broken-up in pieces. The car has 135000 miles on it. The new exhaust made all the differance.
Did they check the emmissions before and after? Was it Dyno tested for performance?
-A

WE find many times catalytic converter failure is from a lean mixture causing extreme heat inside of the exhaust. MOST of the time that lean mixture is caused by carbon build-up on the "film" of the air mass sensor!

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Just curious about how carbon builds up on the air mass sensor if the air mass sensor is in the intake area, upstream of any combustion process. Or are you talking about the O2 sensor?

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Correct! Another good point.
I have found this to be a problem as well. More like a slight coating of dirt
not really carbon. (Looks like carbon build up on the film). I have found that removing and misting the MAF with alcohol will clean the deposits. I will dry it with a hair dryer set on low warm. A dirty MAF will falsely indicate less airflow causing the Computer to lean out the mixture.
A lean mixture will make the cats glow bright red and actually melt the Honeycomb inside. You really must be through when diagnosing a problem.
$$$$$$$
-A

I'm curious, is the MAF sensor upstream or downstream of the EGR? If it is downstream, there's your source for carbon. If it is upstream, how does the EFI know total MAF and how to set the injectors?

Best Regards,
Jim

I could not agree more! I know very little about cars and have to depend on what the mechanic says, so it gets a little frustrating a times. After the regular mechanic I take the car to for regular maintenance was not sure what the problem might be, I took the car to the dealer. When I spoke to them and explained the problem with the CEL, they said the first thing that they would do would be to check the coils (since the spark plugs had just been replaced). And then proceed on down the line till they found the problem. After checking the car, they told me the cats were bad. I asked them if they had checked for other problems too, like something that would be causing the cats to go bad. They said no, as soon as they found the cats were bad, they did'nt look any further. Did not check the coils, etc. When I asked why, they said what they do is fix a problem when they find it and then wait to see if anything else shows up! So I might just have to pay to get another diagnosis done later for the same problem.
Anyway, now I have had both cats replaced (just two days ago) and everytime I drive the car, I keep waiting for the CEL light to go on!!
It may be that the problem is fixed but I don't know for sure. And what if the CEL light does come on, I might have done more damage to the car in the mean time and made these cats go bad too!

where are you located? Perhaps somoene can help with a recommendation to a good mechanic in your area.

If in the Los Angeles Area, I can give you the number of my mechanic

Alon

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