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question !!! is the cat destroied or are you replacing because you got a check engine and code P0120/ P0130 ?????
here's why I ask . I've got an 06 300C 5.7 , 101,000 miles. For the last couple of years the check engine lite has been on alot. just before last years (2018) Mass emission inspection I read the codes and replaced all 4 O2 sensors. got the sticker and was good till it ran out in Sept 2019. This time the codes were for bad catalytic convertors both sides........tried SEAFOAM and a long trip at 70 mph ...lite went out for 2 months....back to the retired person's type of driving and the lite came back on....same cat codes...P0120, P0130.....here's where it get's interesting ~!!! The downstream O2 sensors on these systems , from the factory , had the computer code written to put a very narrow window of voltage to tell the CPM that everything was kosher in the engine !After a while things wear and that GATE get's crashed and the numbers the downstream sensor sends exceed the parameters allowed in the code. Thus the check engine lites and the P0120/P0130 codes. both of which cause you to fail the emissions requirement in Mass . !! So most people literally CHOKE at $ 884.00 per side factory convertors and look for aftermarket one's, at around $400-500 a set. NOW THE FIX !!!!! go to NAPA OR ANY PARTS STORE..ASK FOR THE 18MM SPARK PLUG ANTI FOUL ADAPTERS ,,usually less than 10 bucks !! ,,go home , dig out your drill and bit's and a 7/8's wrench. 6 point is best. jack up the car when it's cold ..the cat's WILL burn you if you try to do this on a hot engine. reach up next to the tranny , disconnect the down stream O2 sensor and squirt the threads with penitrating oil. Remove sensors ! go to work bench , select the drill bit just a size larger than the bore of the O2 sensor, and use it to drill out the center of the 18 MM adapter so that the sensor screw's in. Do both sides and reinstall .! What this does is only allow the sensor to read thru the bottom 1/8 hole ofthe sensor so it sends a message to the CPM that the cat's are AOK. !!!! THIS WORKS !!!!! clear the codes , and watch, or just drive it until the lite goes out. I drove around for 200 miles and then went for the MASS INSPECTION STICKER, no problems .
 
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