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 peformance. 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.
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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 insane 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 idiots have no idea you have no access with the engine on.
Second bit of advice you should not listen on the internet: people just parrot the statement that the catalytic converter cannot be cleaned. I cleaned it.
This is the before picture:
You can see a) the brick with the channels exposed being a small portion of the surface (it was worse off where I did not take the picture); b) the blocked light gray surface which blocks the holes; and c) the scraped off (more on this below) areas (darker gray).
What happens is the light gray ceramic material starts to block the openings. It is very easy to remove, however. I used the wooden end of the brush on the first picture. Do not apply any force, just scrape lightly. Then vacuum from the end you were scraping. Afterwards, put a mask on, and use compressed air from the opposite end of the catalytic converter "brick" to blow out ceramic dust from the channels. Contact me if you will be doing this.
Placed it back: code did not appear again. Listen less to internet advice where people parrot each other.
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