HelloCustomer When you set the switch to AWD, the 4x4 does not engage until the computer senses that the rear wheels are spinning faster than the front wheels. This is the Automatic 4x4 setting. If you want the 4x4 to engage you need to put the switch to 4x4hi or 4x4lo. You can select 4x4hi at any time, but if you want 4x4lo, you need to be stopped and the vehicle needs to be in Neutral.
The P0455 code is for the evaporative emission system. This controls the vapors from the fuel tank. It is very hard to diagnose without a scan tool and a smoke machine. This will need to be done by a qualified technician in order to diagnose properly. It could be a switch, it could be a hose a solenoid. It will end up costing you less in the long run to get it repaired rather than trying to repair it yourself.
The code P0400 is for the EGR system. This is the Exhaust Gas Recirculation system. More than likely the problem is with the DPFE (Differential Pressure Feedback Exhaust) sensor. This measures the amount of exhaust gas that is recirculated into engine for emission control. The problem could be with the egr valve, plugged ports, egr control solenoid, DPFE sensor, hoses. This is also very difficult to diagnose without a scan tool because you need to be able to monitor and activate sensors. My advice is to take the vehicle in and have it either diagnosed and you can replace the parts to save some money, or have them fix it as well. Below are the pictures of both fuse boxes. The one under the hood and the one under the dash.
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