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"no mechanic ever bothered to look into the reason for the CAT going bad."


Why Catalytic Converters Fail

mine is currently throwing a code telling me the converter is bad, I believe my problem stems from a head gasket repair, removing those rusty exhaust system bolts and nuts I may have contaminated it, but that doesn't sound like it would be your problem.

a bad O2 sensor can cause a cat to go bad, either adjusting the mixture too rich or lean, I don't think that was my problem, my problems started just after doing the head repair, no codes before the repair, just a loss of coolant, also I had used brake sytem cleaner to clean the threads in the head, that's nasty stuff, with all the chemicals I used, and my liberal use of them (vehicle had 180,000 miles) and the code coming on shortly shortly after the repair (I think a couple days) I'm pretty sure that's what did my cat in

BTW, too lean causes the temp in cylinder to go up, but in the cat isn't it a too rich mixture which causes the overheating ?
 

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shouldn't the wrong fuel trim or anything else which changes the air/fuel ratio affect gas mileage? and possibly power?

I have kept track of mine for 201,000 miles, since new, in summer it gets 29-31 like always, and now gets 25-28 like it always has in the cold and that they changed over to winter gas

I know what you mean about working on them in winter, MN winter here :)
 
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