Hello! Welcome to the site! Thanks for coming! I'm Ron Z. I'm here to provide as much information and insight as I can, to best answer your question.
The P04423 and P0456 are both directly related. One is "very small" EVAP leak and the other is "small" EVAP leak. Unfortunately, these are very "general" codes simply stating there is a leak in the system, so to give a 1-answer, "it's this..." kind of answer would be impossible. Best, XXXXX XXXXX way to begin here is to have the EVAP System "smoked". This is where a machine forces smoke through the EVAP system and any leaks will reveal themselves with a stream of smoke. Like I say, this is the most efficient way to find a leak, as trying to find a leak "by eye" will be virtually impossible. If the smoke test proves no leaks, next inspect the fuel cap. Check the underside of the fuel cap for any cracks or signs of defects that may keep the cap from sealing correctly. If there are any doubts about the inspection of the cap, replace it. Keep in mind, you will need a cap that meets OE Specs. Those "universal" or "locking" fuel caps sold at after market parts stores do not meet OE specs, and will not seal the system correctly. Check the top of the fuel filler neck for any signs of damage or debris. Check under the vehicle, around the rear areas of the vehicle, looking for any vacuum lines that have dry-rot cracks, holes or loose/missing connections. If the fuel cap is known good, then a very common fail item on these vehicles that "can" set these codes, is a faulty Pruge Control Solenoid Valve. The Solenoid is located on the top, driver's side of the engine. Inspect the electrical connector and wire harness for any loose, broken or damaged wires. If the visual inspection checks ok, suspect a faulty PCSV.
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