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if your getting P0420 the o2 is telling you that the cat is not doing a good job cleaning the exhaust. The cat is on its way out.
You can confirm you have a clogged or restricted cat by hooking up a vacuum gauge. Note vacuum at idle, then increase engine RPM to about 3000 RPM. Vacuum should drop, then steadily climb back up to the reading you had at idle. If it doesn’t climb back up, or it does climb, but drops back down, this would indicate that your exhaust is restricted. A more invasive approach would be to remove the cat, if it’s mounted to the exhaust of course, and to check if it’s clogged.
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