I have a 2003 dodge ram 1500 with a V-8 4.7 engine and my check engine light came on with a P0456 code and I have been told that it could be the gas cap, I have checked it and it seems fine but should I spend the money to have it checked out or should I just buy a new gas cap and see if it goes away!?

The leak in your evap system is very small at .020. It is more than likely a small split in a hose somewhere. The cap is probably ok, we rarely find a bad one. If it is a small split in a hose it will only get larger over time. Here is the factory info on the code. We have found that alot of these leaks are from a small hose on the evap canister under the vehicle, but with out looking at your vehicle it is hard to say. If the leak goes away or is no longer large enough to be detected, your light will go out after a few days. These leaks are emmissions related and will not effect the integrity of the vehicle. I would have it checked and corrected if the light stays on at your convienience.

Symptom List:
P0442-EVAP LEAK MONITOR MEDIUM (0.040) LEAK DETECTED
P0455-EVAP LEAK MONITOR LARGE LEAK DETECTED
P0456-EVAP LEAK MONITOR SMALL (.020) LEAK DETECTED
Test Note: All symptoms listed above are diagnosed using the same tests.
The title for the tests will be P0442-EVAP LEAK MONITOR
MEDIUM (0.040) LEAK DETECTED.
When Monitored and Set Condition:
P0442-EVAP LEAK MONITOR MEDIUM (0.040) LEAK DETECTED
When Monitored: Immediately after a cold start, with battery/ambient temperature
between 40° F and 90° F and coolant temperature within 10° F of battery/ambient.
Set Condition: If there is a leak larger than 0.0409 and smaller than 0.0809 in the
evaporative system.
P0455-EVAP LEAK MONITOR LARGE LEAK DETECTED
When Monitored: Immediately after a cold start, with battery/ambient temperature
between 40° F and 90° F and coolant temperature within 10° F of battery/ambient.
Set Condition: There is a leak larger than 0.0809 in the evaporative system.
P0456-EVAP LEAK MONITOR SMALL (.020) LEAK DETECTED
When Monitored: Immediately after a cold start, with battery/ambient temperature
between 40° F and 90° F and coolant temperature within 10° F of battery/ambient.
Set Condition: There is a leak larger than 0.0209 and smaller than 0.0409 in the
evaporative system.
POSSIBLE CAUSES
LEAK AT GAS CAP
VEHICLE HISTORY AND SERVICE BULLETIN INVESTIGATION
VISUAL AND PHYSICAL INSPECTION
VERIFY EVAPORATIVE EMISSION SYSTEM LEAK
EVAP PURGE SOLENOID OPERATION
INTERMITTENT LDP MONITOR FAILURE
EVAPORATIVE EMISSION LEAK DETECTION
GAS CAP
LDP COMPONENT LEAK

You can ck the cap by listening for pressure release when you open it. A could be is a wild guess at your expense.No, verify the defect first.