Q: P0420 code after idle relearn

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I need help diagnosing a p0420 code. Background- 20k-30k ago bad cats blew the motor. All cat replaced and have 20k-30k on them, used motor has 120k. A while ago a throttle body clean was done the wrong way I guess (not by me) and I had to re-learn the idle which took about a week of trying. The day that I got the idle back to normal is the exact same time the p0420 popped up right after messing around with the relearn procedure. Still to this day haven't figured out what could cause the code. Since the cats are fairly new I keep telling myself it can't be that. Other causes I've heard are bad ecm program (which would make a lot of sense given the circumstance) or exhaust leak (Which I have had a minor flex pipe leak for a good while now). What are the chances I destroyed my ecm and what are the chances it could be as simple as replacing the flex pipe?

My car has 140000 miles.
My car has an automatic transmission.

Hello. P0420 is the code for catalyst efficiency below the correct threshold for bank 1. If you do not believe the catalytic converters are the issue, then the first thing I would do is check the bank 1 oxygen sensors. It may be possible that the downstream sensor is lazy or no longer accurate and and setting off the code. If the oxygen sensors are good, then the next things I would look at are the computer and the catalytic converters, even though they maybe new. If you would like to have an expert scope out the problem, a qualified professional from YourMechanic can come to your car’s location to diagnose the Check Engine Light and pinpoint the correct repairs.

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