2004 Cadillac SRX - STS Catalytic Converter / Engine Light

The engine light on my 04 Cadillac SRX has been on the last several days. I have owned it for several months and am not sure what fuel octane was used but I use regular 87. The manual said to use 87 or higher such as a midgrade. The manual also says if the engine light is on steady, then there is an 'emission control system malfunction' which may be able to be corrected by reinstalling the fuel cap, checking for a loose fuel cap, or changing brands of fuel. It said the steady light may go away after a few days after taking these actions, or after at least one fuel tank change with a different gas. I did change the fuel one time and the light did not go away. I took it to my mechanic to put it on his diagnostic machine and he thought I may need new catalytic converters. He said to change to a higher octane first and see if that will eliminate the light. He turned the light off and said to stay with a higher octane but if then the light came on again that I will need to have the 2 catalytic converters replaced. He said GM would do that for free under an 80,000 mile warranty on the converters. Any suggestions?