I have a 2013 MKZ, I had to change the converter on the firewall side at 82,000 miles. At 100,000 miles, if my memory serves me well code P0422 tripped, at least I think that was the code. I know we ended up changing the same converter again. Well, here we are again at 116,000 miles and code P0420 Bank 1 just tripped. I would like to know what side bank 1 is on and if it's the same one. What could be causing it to fail? Do you think I'll have to replace it again or could it be a sensor or something else that is causing this issue?
You have to find out what upstream is causing the failure. Is the engine burning oil due to worn out piston rings? Is the PCV valve clogged and causing oil burning that's contaminating the converter? How about the injectors leaking causing an overly rich air/fuel mixture? Are your replacements OEM or after market parts? First thing I would do is get a compression test.
