96 Chevrolet Beretta 4-Cylinder P0301 Diagnostic: Causes and Solutions

What could be causing a P0301 fault code on my 1996 Chevrolet Beretta with a 4-cylinder engine? I've already replaced the spark plugs and ignition wires, so those aren't the issue. Could there be another underlying cause? Any insights or troubleshooting steps for this specific OBD2 code on a 1996 Chevy Beretta?