Brought it in and the code that was given sugested change gas cap which we did but engine light is still on. the code given also suggested vacume leak or replacing canister. the car does not miss or idle roughly the engine seems to be working fine. what should we check for first. the purge solenoid vale and where is it located and how do we check it. if that shows good then do you suggest changing the charcoal cannister and is that located by the gas tank
What code ? P0442 or P0456 or p0455
DTC EVAP P0442 P0455 P0456
EVAP Leaks P0442 and P0455
Wells EVAP Training with Sgt Tech P0440 P0441 P0442 etc
SOURCE: 2004 Pontiac Grand Prix Evaporative Canister
you should be able to fix it look for a can near the fuel tank that has vacuum hoses coming out of it and you should be able to just buy the bad solenoid
SOURCE: 1992 pontiac grand am no acceleration, a rough idle (engine miss)
Does it have the 2.3l Quad 4 engine? If so replace the plastic piece that the coils go into, they go bad and leak spark to the cam towers and then you get a nice one or multiple cylinder misfire.
SOURCE: changed the gas cap,evap canister, and the vent
you need to check the vacuum hoses and fittings going back to the gas tank. you have to make sure there are no cracks in the hoses and make sure all the connectors have good o rings. leave comment
SOURCE: My 2004 Sunfire check engine
DTC P0442, P0455 or P0456 are all diagnostic codes indicating a leak has been detected in the EVAP system:
- P0442 EVAP Leak Monitor .040 Leak Detected (Medium Leak)
- P0455 EVAP Leak Monitor Large Leak Detected
- P0456 EVAP Leak Monitor .020 Leak Detected (Small Leak)
What this code means is that the computer has tested the evap system and was not able to draw a vacuum on the fuel tank. it does this by turning on the evap vent valve which closes the fuel tank vent off, It then opens the vacuum solenoid and allows engine vacuum to be drawn on the fuel tank. A sensor in the fuel tank monitors the Pressure/Vacuum levels on the tank and reports this to the computer.If the computer does not see a change in pressure/vacuum on the tank then code P0455 will set.
Check under the hood to make certain no vacuum lines are disconnected, check your fuel cap if it isn't sealing correctly or is missing or the wrong one this will cause that code to set, You may have a bad vent valve in the gas cap causing the problem.
If you first get a P0442 or P0456 then a while later get a P0455 it is a sign a hose is going bad.
It is common for the hoses over the gas tank on Jeeps and some other chrysler cars to go bad, which requires removal of the gas tank to repair. Could also mean a piece of charcoal from the vapor canister is stuck in a line. Blowing the lines out can fix it.
If you get a P0455 (large leak) at the outset it more likely a stuck valve or a hose which came loose. A smoke machine is the fastest way to finds leaks in these systems but not a common tool for do it your selfers (click image diagram for zoom). 
Additional details could find in this TSB procedure DTC P0440 (click over link).
If you have additional comments about this, keep in mind also send us exactly what faulto code found there.
Keep us updated.
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