I get it too, and I code read it while it was still running with the light on, and it gave me VSS code, which is to do with the vehicle speed signal. It tends to come on if I'm sitting just over 3k rpm for a few mins. It always clears after turning the car off. I've heard of plug n play ecus throwing up random codes before, but It doesn't bother me as the car runs no different when it's on and my AFR gauge reads fine, so I just ignore it, but I suggest you try reading it before you switch it off next time just incase it is something important.