2018 equinox 1.6 diesel fwd. 73000 miles. Did oil change the other day and drove it 10 miles came home no leaks -level good closed it up. Wife went to work 2 days later and got a half a mile down the road and the check engine light came on. She came back home and took the spare car to work. I had the code pulled and it is p-0421 warm up catalyst efficiency . Vehicle has been trouble free since new and fluids and filters have been replaced regularly. i searched and only came up with 1 or 2 threads that have had this code.One went away on its own with no action from the owner i believe. I spoke to mechanic i know and he said this code can be a pain to diagnose. He suggested using seafoam in the tank of new fuel and that a lot of times a bad tank of fuel can cause this to happen .He suggested clearing the code a few times and hopefully it would go away. I am just looking for anyone who might have had this code come up and how they fixed it. I know if i go to dealer they will do the 2 recalls that are computer related and clear the code and send me on my way .