I had this problem on my WRX forever. It is a small emissions leak. It seemed to mainly come on when I was cruising on the highway. I noticed it once I changed my coolant to turbo hose because the stock one melted on my big tmic. After getting the cel......then it turns off by itself........then comes back......then back off and back on....etc. I replaced the hose with another stock one and never got the cell again. This cell is probably one of the hardest to detect where it's coming from. Have you replaced any lines,hoses, or sensors on the car? What mods have you done? good luck.