P0456, check engine light and torn fuel cap gasket
 

I started getting a check engine light a few weeks ago (2018 BRZ with 20,000 miles). Bluetooth scanner showed P0456 - very small leak in evap system. Checked the fuel cap (which looked fine), tightened it and reset the code.

The code would come back after around 80 miles and a few days of driving. Visual inspection of the easily accessible evap components didn't show anything suspicious, so I decided to take a closer look at the fuel cap.

I suspected that perhaps the gasket has some tear or missing piece that caused it not to fully seal. Pried it out, then saw there's another gasket under the first. Pried that one too. Turns out its the same gasket, just completely separated the two halves.

Couldn't find a way to order just the gasket, so purchased a fuel cap replacement from Subaru (42031FJ000) for $35.

Replaced the fuel cap and no more check engine or fault codes for over 300 miles.

Any idea why the gasket failed so catastrophically? Would some silicone grease applied to the gasket help prevent this?

In the pictures, new fuel cap on the left, old one on the right (with little visual difference in the gasket at a cursory inspection).

Gasket in my hand is the new one, two gasket pieces on the table are the old gasket that failed.

RKX makes one that works pretty well.

https://smile.amazon.com/gp/product/B07TYGLPPQ


https://live.staticflickr.com/65535/...05abb1_z_d.jpg

I replace my gas cap (accident with a guard rail) and it didn't help but this gasket has made it better. I still get the code every 3-4 weeks or so and I just reset it. The shop claimed to have smoke tested it as good but I know I'll eventually end up buying a smoke tester and testing the evap myself. Hopefully this one fixes your problem completely.