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Cleaning the catalytic converter
NOTE: In most cases, it is not possible to clean the catalytic converter to get rid of the P0420 code. In most cases, it has to be replaced, along with the oxygen sensors. In some cases, you can TEMPORARILY get rid of the P0420 error if it came on recently by taking the catalytic converter off your car, and cleaning it gently with compressed air, and cleaning/replacing the oxygen sensor. Do not listen to online advice about adding thinner, or any other liquid to your gas tank. That will never clean your catalytic converter. NOTE: you will also get decreased engine performance anyways with clogged up oxygen and MAF sensors. It is still necessary to clean or replace all of these parts to have the engine operating at peak performance. There are no magic cures to the failing / old catalytic converter. You will need to replace it sooner than later. You may save money by finding an EXACT OEM complete catalytic converter or one you can have welded to your exhaust system, BUT IT MUST BE EXACT SIZE AND SHAPE.
After I have replaced the oxygen sensors, which were covered with soot and other deposits, the car no longer misfired on the highway. It did, however, throw up the dreaded "Catalytic converter below optimum efficiency" code.
I went online to see how much a generic replacement and whether it can be cleaned.
First of all, stay away from generic catalytic converters which must be welded by you into the old pipe. They list generic crap online which will never fit because of length, angle, and several other factors. For example, this is a picture of mine catalytic converter:
There's no way a welder can cut it out and weld a new one to those pipe ends at these angles.
Second bit of information: unless you are patient and/or know what you are doing, do not attempt to take the catalytic converter off the car. It is nearly impossible to access the bolts. I had to use some very weird socket extensions, have two people, et cetera. And all manuals and online advice stated dryly "take the catalytic converter off the car". Those people have no idea you have no access with the engine on.
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