Hummer, what vehicle specifically are we dealing with here? This will help us narrow down your problem to a very specific thing. With that being said, P0456 is your Evaporative system leak error (small leak). So, in short, what this is saying is that the vehicle is unable to obtain a vacuum on your gas tank to prevent the escape of gas vapors. So, odds are, if you started your car with the gas cap removed, that will cause this error code to trigger or you refilled your vehicle with the motor running. After that, having a gas cap that doesn't seal well will also cause this to come in. I would recommend first replacing the gas cap and then resetting the code. If the code comes back, then you will need to get under the car and start looking at the lines running off of the evaporative canister. There is also a vacuum break valve on the side of the canister that can also get a little bit of dirt in it and cause the valve not to be able to seal properly. If replacing the gas cap does not fix the problem, then let me know.

 

Hummer, well, then you next step will be to get the back end of the car up in the air and start doing a hand over hand of the hosing running from the filler tube to the gas tank, the gas tank to the evaporative canister and all the hoses running off of the evap canister. Gently bend the hoses and if you see any cracks in the hosing, replace that section of hosing. If you look at the evap canister, you will see a 2 wire connection on it. Those wires go to a solenoid valve. You should be able to pull the valve off of the canister and then clean the inside of the valve.

Unfortunately, this is one of those problems that will require a fair amount of time to simply look at the hosing and get a feel for what is possibly bad.

Another thing to keep in mind is that the engine needs to be started, driven for about 3 (or more) minutes and then turned off 5 times before the check engine light will clear. During these 5 drive cycles, the fault can not be present or else the counter is reset back to 0. You can bypass the 5 drive cycles by disconnecting the battery for a few minutes and the reconnecting. This clears the memory in the ECM. But, before you do this, make sure you have the code for the radio as this will also toss the radio into a protective mode and the radio will not function till you put the proper code into the radio.

 

Great stuff Thermo
any update Hummersa?