Hi there,
I had this problem for a while with amber little engine Dickyn, first at all i have got EOBD-facile App with ELM327 WIFI diagnostic tool, easy and cheap as apps costs £17.99 and tool around £10.00, then showed problems with lambda sensor, then something wrong with exhaust, but sometimes is just sensitive and appears without any issues!!! I found out that previous owner used car only once in the while, so needed a time to clear out.
Also Apps is easy to clear all your DTC's and trouble codes.