Can drive — fix at your convenience.
P0456 indicates a very small leak in the EVAP system, the tightest leak the system can detect. The most common causes are a gas cap that's slightly loose or has an aged seal, or a tiny crack or pinhole in a hose. Because the leak is so small, it can take a while to track down, but it has no effect on how the car runs.
$20 – $250
Varies by vehicle and root cause.
Frequently, yes. For a very-small-leak code like P0456, the gas cap is the first thing to check. Tighten it until it clicks; if the light persists after several drive cycles, a fresh cap ($20-60) is the cheapest fix to try.
Absolutely. It's a tiny vapor leak that has no impact on drivability or safety. Address it when convenient so the light clears and you can pass emissions.
A gas cap runs $20-60. If a hose, seal, or valve is at fault, repairs usually total $100-250 with diagnosis, since tiny leaks can take time to find.
No, it's low severity. The leak is very small, won't harm your engine, and won't strand you, but it will keep the check engine light on and fail an emissions test.