I was under my truck today just admiring it and found a black canister round in shape on the outside of the frame under the passenger side along with a black box next to it. It is about at the back wheels but close to the bed/cab connection area. What is it?

 

Fuel Vapor (aka Charcoal) Canister

Definition:
This is the main component of a vehicle's evaporative emission control system. This system controls the escape of fuel vapor from the fuel tank. The charcoal canister stores the vapor until it can be burned in the engine at an appropriate time. All vehicles built since the early seventies are equipped with a charcoal canister.

 

could that be why my check engine light is on?? its code p0455 i think.. a evap problem.. they said it was the gas cap and fixed it.. and the light actually went out for 5 hours then came back on :-( and its still the sammee problem

 

Is your gas cap on tight? 3-4 clicks?

Otherwise, it could be your evap canister.

Sime Diagnostic Info...also, consult the factory service manual available for download in the "merry christmas" thread.
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During evaporative system diagnosis, Leak Detection Pumps have mistakenly been replaced. A thorough inspection for pinched, kinked., or disconnected supply vacuum lines (as currently listed in the Powertrain Diagnostic Procedures) is a critical step in diagnosing DTC P1494 (LDP SW OR MECHANICAL FAULT). Additionally, the other evaporative leak faults (DTC P0442 - SMALL LEAK, P0455 - LARGE LEAK, and P0456 - VERY SMALL LEAK) should have the supply vacuum lines examined for pinches or kinks prior to any LDP component replacement.

The supply vacuum lines should be checked from the engine compartment all the way to the fuel tank, including the LDP and purge system.

NOTE: VERIFY ALL RELATED SUPPLY VACUUM LINE ROUTINGS FOR PINCHES, KINKS, OR DISCONNECTION'S BEFORE REPLACING LDP SYSTEM COMPONENTS.

 

I don't know what the PCM code was, but my charcoal canister had to be replaced at like 62k miles. The tech did a leak check on damn near the whole truck before locating the problem......Check the vac lines and stuff first like Burning Rom said. The charcoal canister was not a cheap item to fix, it was over $100

 

So if the if there's a leak in the vacuum lines or the charcoal canister isn't working properly, it's not harming the vehicle is it? It's pretty much just something that tree huggers came up with so we don't "burn a hole in the sky", right? My neighbor's '02 Durango is showing error code P0441, which has to do with an evap leak of some sorts. It's not the gas cap - check it, that would have been to easy.

 

No way! Awesome!