Can drive — fix at your convenience.
P0455 indicates the EVAP system detected a large (gross) leak, meaning a significant amount of fuel vapor is escaping. The biggest and easiest cause to rule out is a gas cap that's missing, loose, or has a bad seal, but it can also be a disconnected or cracked hose or a stuck-open valve. It doesn't usually affect performance, but the leak is larger than the small-leak codes.
$20 – $350
Varies by vehicle and root cause.
Surprisingly often, yes. A missing or loose gas cap is one of the most common triggers for a large-leak code. Make sure the cap is on and tightened until it clicks first, then a replacement cap ($20-60) before any larger repair.
Yes, in most cases. The car runs normally and it's safe for normal driving. You might smell fuel, though, so check the cap immediately and get the system inspected soon.
If it's the gas cap, just $20-60. A cracked hose, stuck valve, or canister repair generally costs $100-350 with diagnosis and labor.
It's still considered low severity for drivability, though the leak is bigger than the small-leak codes. It won't hurt the engine, but it releases more fuel vapor and will fail emissions, so fix it promptly.