I have a similar strange one on my Impreza in August and September I was getting a P0420 code every 20 miles or so.
Since the temperature has gone down below 10C not one error code
Well thats the catalyst not working right code which could either be the catalyst or the second lambda sensor. Not obvious why temperature should affect this - it could even be that you drive differently when its colder. Either way the sensor or cat are on the edge and higher termperatures tip it over.
