Catalytic Converter warning light
 

Hi
The catalytic converter warning light has just started appearing on my dash.
She has been coughing and stuttering for a few days now.
Is there a recommended additive i can add to my fuel tank before spending $$$ on a mechanic?
Just moved to Cambridge and we've had some really heavy frosts. Problem started after that.
I do have antifreeze in my radiator btw.
Mitsi Diamanti 1995 Had her 10yrs with no problems.
Cheers