my check engine light is on had it check machine said needed new gas cap repl

Code p 0456 is small evap leak and they say it's the gas cap but, I replaced the gas cap and light came back on 5 days later. Where and how do you get the evap system tested?

i would check the evap system, vent solenoid and hoses you have a good chance leaking somewhere there

The EVAP system is spread from the gas tank to the engine. The code you had was probably for a 'large leak detected' and ONE of the possibilities is the gas cap , but that is as you've found not the only poss. cause. Less aggravation maybe , to have a qualified shop diagnose it. EVAP system leaks can be pesky.