2003 Buick Lesabre P0480 Code: How to Fix and Pass Emissions Test

I need to pass my vehicle's emissions test, and the car runs perfectly fine under normal conditions. The P0480 diagnostic code has been persisting despite no noticeable performance issues. The cooling fans are functioning correctly and have already been replaced. I've noticed that the AC takes a while to reach cool temperatures and sometimes blows slightly warm air at highway speeds—this may indicate low refrigerant levels. Since I purchased the car two years ago, I haven't addressed the system maintenance, which might be contributing to this issue. The P048 or P0480 code is related to the AC system's pressure sensor and can trigger emissions failure even though it doesn't directly affect tailpipe emissions. I'm concerned that this code is being flagged unnecessarily, but I need it resolved to pass the emissions inspection. What steps should I take to clear or fix this code?