I have a 2005 Subaru Impreza. Recently I started getting a CEL shortly after getting gas. I know that the cap could be the cause for this but I wasn't sure. I could remove the cap and put it back on and sometimes the light would go out in a day or two. Next time I got gas, the light would come on within a day. I ran a scan and although I don't recall the actual code that came up, I remember it being gas cap/emissions related. Now I have a full time light so I decided to get a replacement gas cap. I went to NAPA and bought one from them. I was told that the Stant caps they sell are OEM quality so I figured I would go for that since the nearest Subaru dealer for me is... well, there is no nearest really. Anyway, 3 days after replacing the cap, I still have the CEL and now I'm wondering where to go from here.

Subaru genuine OEM cap?
One of the evap vac lines?
Something else?

Did you bother having someone scan the codes yet? And if not why not? You will get a very good reason for it from the code or codes that have been tripped in the ECU. Right to why and what subsystem. Its ODB2 so you don't need to go to Suburu to get it done.

It will be amazingly specific in what's wrong.

I mentioned in the post that I ran the scan myself but couldn't recall the number off the top of my head. I just went out to the garage and found the information...

Code was P0457- EVAP System Leak Detected Fuel Cap Loose/Off

Have you been pumping in additional gas, after the pump cuts off?

I mentioned in the post that I ran the scan myself but couldn't recall the number off the top of my head. I just went out to the garage and found the information...

Code was P0457- EVAP System Leak Detected Fuel Cap Loose/Off

I missed reading that in the post...

Check the lines running to the evaporation canister under the hood to the tank... it appears there is a vacuum leak in the fuel system... if you replaced the cap.. its likely someplace else... and that's the next most likely place to look.

Make sure they are tight... and not loose, cracked or otherwise damaged.

Have you been pumping in additional gas, after the pump cuts off?

Are you kidding? With the price of gas? LOL... no, I only go about $30 which is nowhere near full.

Based on what I've read here, I'd get an original equipment cap, clear the codes and watch for the light (if it comes back).
Many aftermarket parts will fit but they don't quite match. With a P0457 small leak detected, even the slightest leak can cause the evap system to turn on that light. Always turn the fuel fill cap till you hear the clicks to ensure it's seated.
Let us know if that code comes back.

Thanks. I will order am OEM cap from Subaru.