Hi Scotty!

I know little about cars, but have learned more than I NEVER wanted to know recently. I have watched and enjoyed your You Tube channel (especially the flailing arms!!). I have had a 2015 Buick Encore  automatic with 106,000 miles and the infamous 1.4 L Turbocharged engine for one year. I recently had  a PO299 code appear and had the turbocharger replaced. It took three go arounds. The first two broke. Then I was told the PCV valve was plugged and I needed to replace the intake manifold also. I'm sure you know that on this car the PCV valve is part of the intake manifold. I also decided to have the valve cover with diaphragm and PCV hose replaced at the same time, since those are common issues with this car.

I got just a few miles away from the repair facility, when the check engine light appeared again! They told me they test-drove the car and everything was fine when I picked up the car. This time there was a PO420 code. Now I was told that suddenly the catalytic converter was bad and needed to be replaced. They checked for leaks and faulty O2 sensors. My warranty company told me I needed a new engine (sludge). They paid for the turbocharger replacement but not the other repairs. 

My question is what would have caused the catalytic converter to go bad AFTER all the other work? I have used lacquer thinner (twice) per your advice as well as Cataclean and CRC. I have also been using Marvel Mystery Oil as a fuel treatment with each fill-up.  The PO420 code remains. I removed a quart of oil and put a quart of MMO in the crankcase, too, as a gentle cleaner.  The car is running great, especially for one that supposedly needs a new engine!!  The gas mpg have improved also.  I have an OBDII scan tool and can erace the code that way. I can drive for hours on the freeway at high speeds and the check engine light will stay off.  As soon as I drive 5-10 miles on city streets, the check engine light is back on.  Why?? I don't want to have to replace the catalytic converter at this time.  Any helpful advice would be greatly appreciated.

Thanks for all you do and have done for vulnerable car owners!!