I assume you really means 300k miles, and not 300k km. This does implies the age of catalytic converter might need to change. However, if you would like to start cheap, try check and change the following in sequence:
1. Engine oil and oil filter, if you have not change after 3k miles. Flush the engine
2. Air filter. Ensure it is clean. Replace if dirty
3. Spark plug. Check and replace if require
4. Clean distributor cap. Ensure not crack and fully functional
5. Spark plug cable. Ensure no crack and current leak
6. Acetone suggested by Dru M can clean up fuel system indirectly as well. Try add in and drive for half tank. Cost is about $6
7. If all done, and still failed catalytic efficiency, replace catalytic converter. Save the environment is every human being's responsibility
My 1992 Honda Prelude nearly fail catalytic efficiency as well. Tried all 6 solution above, and finally replace it with high flow catalytic converter