P0455 FORD Possible Causes
What does this mean?

• Missing or loose fuel cap
• Incorrect fuel filler cap used
• Fuel filler cap remains open or fails to close
• Foreign matter caught in fuel filler cap
• EVAP canister or fuel tank leaks
• EVAP system hose leaking
• Fuel tank leaking

Best check the pipes from the cap down to the tank for rust through.
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First the obvious, look, see, replace what is broken. And of course do internet search of the code. Like I did.
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P0455 Indicates that there is a large leak in the evaporative emission control system (EVAP).

The car is failing the evap test because of a large leak. When you are driving, the car puts a vacuum on the gas tank and than closes the valve on the purge solenoid and sees if it can hold a vacuum. If it can't, depending on the size of the leak it will throw a large leak or small leak code.

You evap system is not able to hold a vacuum. You can try testing it by pulling off the purge valve and applying a vacuum to the hose and seeing if it holds. If it holds pressure, your purge valve is bad and not closing. Possible temp fix is to smack it a few times and spray some WD-40 or another similar type into the valve.

If the tank still loses vacuum check the gas cap and listen for air being sucked in. If the cap is good, one of the lines that the evap system uses or the tank is probably leaking some where causing the issue.

 

The gas cap has valves that prevent too much pressure and release that, likely only in a fire. Another valve that prevents vacuum in the tank and lets air in, but that would be rare as gas makes "vapor pressure" at normal temperature. The car expects that very low pressure at all times except when you gas it up. If it fails to build some vapor pressure after a fill up, then you have a large leak or the cap is left off or loose. Any hole in the closed system will make for non pressure from vapor pressure and sets a light. Some cars can tell the difference between a cap off and just a large leak. Caps are important and best stay with a brand name, or as for me a JY cap. I recently got a JY cap to replace the one on '05 that did not have a tether. Also had a Lin Cont have a cap break in pieces when removal to gas up. JY cap lasted rest of the life of the car for me. Lin and up to G-4 Bulls same cap.
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