Newbie here. Just thought I'd share my experience resolving Trouble Code P0420 on my 2009 Aveo with the LXV engine, with 74,000 miles (118,000 km).
Some history: In January, I replaced the engine oil cooler because it was leaking oil into the coolant. To replace the engine oil cooler, you need to remove the manifold converter. Because the 3 bolts connecting the manifold converter to the flex pipe were badly rusted and likely to snap, I opted to replace the existing manifold converter with a new Walker manifold converter from RockAuto. Although it was a frustrating job to replace the oil cooler, i eventually got it put back together and running. It looked and ran good. New engine oil cooler. New Walker manifold converter. And new upstream AND downstream AC Delco oxygen sensors. The car ran beautifully.
About 2 weeks after the above successful fix, I get the dreaded P0420 Trouble Code, which normally means the catalytic converter is failing. That was a real head scratcher for me, because the Walker manifold converter was brand new (good made-in-USA quality too), and the O2 sensors were new. I had installed everything carefully, so I knew there was no exhaust leak at either end of the manifold converter. The car was still running beautifully. I thought it was maybe an intermittent fault, so I cleared the Trouble Code with my little code reader. However, a couple days later, the CEL was on, and it was again P0420. I kept clearing the P0420 code, and it kept coming back within 1-3 days. I must have cleared it 10 times the last month or so. This was getting very frustrating. But i knew it was extremely unlikely that the converter was the problem.
A careful review of the freeze frame data from the code reader, and studying my Helm service manual didn't help much. I spent hours Googling P0420 posts and found an interesting post by some guy with a Chevrolet HHR and the 2.4 litre engine. He said he was able to solve his P0420 problem with the replacement of all 4 fuel injectors, because he suspected a leaky injector was causing too much unburned fuel into the catalytic converter, which would trip up the catalytic monitor test His story caught my interest, because my fuel injectors had become very noisy during the last few months. The noise sounded almost like noisy valve lifter tappets. And that injector noise would come and go without any apparent reason. I was wondering if the injectors might be going bad. But the car always ran fine so I didn't think much of it.
So believing that one or more fuel injectors could be faulty, I finally bit the bullet and ordered 4 new AC Delco fuel injectors (55559377) from my local GM dealer. I got them for $143/each canadian, for a grand total of $648 canadian including tax. After a frustrating 2 hours removing and replacing the injectors, and clearing the P0420 code one last time, the car was up and running. (By the way, the old injectors seemed to look okay when i removed them). Therefore, I am happy to report that after a week of driving to/from work, including some decent miles up and down Hwy400, my frustrating P0420 code is finally gone.
So there you have it. In conclusion, while a P0420 code usually points to a failing catalytic converter, that is not always the case. Because of my unique circumstances, having replaced the converter and oxygen sensors only weeks before, I knew the problem had to be somewhere else. For my 2009 Aveo, the problem turned out to be one or more failed fuel injectors.
(By the way, in case someone thinks I should have first run some high percentage PEA fuel injector cleaner before replacing the injectors, yes I had already done that too. Over the last few months, I had used several bottles of CRC G2P, but it didn't help this particular matter.)