Hi Grand Cherokee 2002 V8 has a code 0456 and gas cap is good.

Question

What can be done to find where the real problem is?

Answer 1

P0456 EVAP leak monitor small leak detected

A EVAP machine should be used in this case, this will ensure the system has no small leak (.020). Chrysler has produced a TSB bulletin for this known condition TSB # 18-002-03. which involves erasing and reprogramming the PCM with a new software.