2004 Chevrolet Malibu P0420 Code: Causes, Symptoms, and Diagnosis

I'm experiencing several issues with my 2004 Chevrolet Malibu, and I'm trying to understand what the P0420 diagnostic code and other symptoms might mean. 1. I've had a recurring problem where antifreeze is evaporating from the top right side of the engine, likely due to a deteriorated gasket. This has led to low coolant levels once a month and occasional overheating. Replacing this gasket is estimated at $800. 2. Over the past few months, I've noticed a strong gasoline odor coming from the front of the car while driving—though it hasn't affected fuel efficiency. 3. The vehicle runs smoothly under normal conditions until recently. 4. Two days ago, the car would not start at all. It turns over fine and shows good battery voltage, but still won’t start. 5. A diagnostic check revealed code P0420 (Catalyst System Efficiency Below Threshold). I'm seeking clarity on whether these symptoms are related or if one—like the P0420 code—is linked to a fuel system issue or another underlying problem. Any insights would be greatly appreciated.