2016 Outlander SE, 136k miles. For 2 years now my service engine light will come on. Sometimes it stays, sometimes it will randomly go off. The code it throws is P0421 which obviously points to the cat, but I am not so sure that's the issue. Fuel efficiency (by me calculating not the dashboard) is averaging 27-28 mpg. If it was solely the cat I would assume my fuel efficiency would show that. I changed the O2 sensors twice. The first time I changed them the light stayed off over a year. So when it came back in I changed them again. It stayed off for a while but has come back on and stayed on now (although sometimes it does randomly go off which allowed me to get it inspected). But to add to the odd situation, I took it up to advanced auto to get the code read in December. I wanted to make sure it was sending the same code and I was going to replace the gas cap. When he hooked it up he got the same codes and I went in and got the gas cap. As soon as I got in to leave, I couldnt push down on the brake and a new light popped up and error code about my ABS and I literally couldnt start the car. So he came out and said he would check my battery and said it was reading low (which is possible but why ALL of a sudden is it a problem with no other signs). So he jumps me and it started. I drove straight home and looked and there was corrosion on my battery. So I cleaned it off and all lights went off, even the service engine light (but eventually came back on). So now I am wondering is the battery bad and causing electrical issues? Would that be why the light is coming on? I have no other indicators that the battery is bad. No hesitation when starting the car or anything. I am just at a loss and hate to replace all this and STILL have to have a new cat conv put in.