P0300 Saturn - Outlook What is wrong with code p0300?

Question

The code p0300 is present and the mechanic performed the inyectors service, but the problem is still there, I'd take it to the Chevy dealer for a diagnostic and they told to replace the catalyctic converter (left side),what I did, the car run good for almost 2 weeks, but after that it started to have the same problem, my mechanic suggested that probably the other catalytic needed to be replaced as well, i did, but the problem still there (code p0300 and the engine is trembling). Please help, I've spent a lot of money.