Q: I'm getting a check engine light with codes p0455 and p0456. I tried a new gas cap from the dealer but the light comes back in a few days.

Hi there. The fuel cap could have been one item that may have been leaking. There could be a leaking return fuel vapor line or the purge vent valve could be damaged. Check the fuel system and look for any hoses that could be cracked or missing. Also, check the purge vent valve on the charcoal canister behind the fuel tank and see if it has a leak or if its not working. The computer is picking up the fuel vapor leak and setting the engine light with the codes P0455 and P0456. If you need further assistance with your fuel system, then seek out a professional, such as one from Your Mechanic, to help you.

Hi there - the 2 codes you mention are indicative of a leak in the evaporative emissions system. As you have been advised, this is sometimes caused by a loose gas cap, or one that no longer seals properly. It can also be caused by cracked or leaking hoses in this system. It can also be caused by leaking vent valves which do not allow pressure leak testing which is done automatically during each driving session. I would recommend an emissions system inspection by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs.