So you're getting a P0446 and a P0456?
Evaporative Emission Control System Vent Control Circuit
Evaporative Emission Control System Leak Detected (very small leak)
What it also says for the P0456:
A 0.020 leak has been detected in the evaporative system. See TSB's 25-001-02A, 18-002-03 and 25-004-01. Also, possible faulty or loose gas cap.
(Also see Notes in P0455 above). Does customer regularly commute on a steep grade (down hill grades in excess of 10%? Has the Leak Detection Pump Operation and Diagnosis booklet (Publication No. 81-170-000606) been used to assist in the diagnosis?
Review and if necessary, perform TSB 18-010-02. Be sure to completely read the TSB. The fix for the condition is a PCM replacement, not a reprogramming of the controller. Be sure that there is not any leaks in the system. This TSB applies in a very limited number of cases and usually in mountainous regions. TSB 18-010-02 Is a PCM replacement correct an erroneous MIL illumination for SAE P0456 that sets when driven on steep grades.
All informatino gathered from www.wjjeeps.com
My question is: Have you seen if any TSBs have been performed on the vehicle?