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The P0456 is "EVAP System Small Leak Detected". This is a very tough one to diagnose. The best method to diagnose this, is to have the EVAP system "smoked". This is where a machine forces smoke through the system, and the "leak" will reveal itself. Prices vary for this service from shop to shop, but it's the most efficient way, as trying to find a "small leak" in the EVAP system can drive you crazy! There are a few common things to check before the smoke test:
Make sure the fuel cap you replaced meets Chrysler OEM specs. For example- those "universal" or "discount" caps will not meet specs, and will not seal the system. The Purge Solenoid is also a fairly high rate fail item.