Why am I eating Cat. converters in my truck? GOING BAD

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Hi,I hope you can help! It start about a one and half yrs ago. First time I repalce the 3 senors and the two cats. converters,grant you the first time I change the cats. It was from a changes shop. (Miekie) But 5 months ago I change the cats. converters only with walker OEM parts very good parts, with a 5 yrs warrenty. Miekie cat went bad. Now the walker are bad too, I take very good care of my truck,tune it up about 25,000 to 30,0000 mile, oil changes 2,000.The truck runs very good, no miss fires or anything like it. Toyota dealership says,engine is fine.Can you give me a reason what is wrong. HELP!!!!! Joe!!!! The truck have about 115,000 miles on it