The Steam error e502 l3 often appears when something goes wrong when you try to buy something on Steam.
It often plagues the Steam storefront whenever the service is overwhelmed due to tons of users, which usually happens during big Steam sales.
If you suspect this is the case, you can always check at sites such as SteamStat or SteamDB. SteamDB is a handy site that will display whenever Steam has problems or its servers go down.
It will also display multiple servers from multiple regions of the world. So if you see your area among the list of down servers, chances are the servers are the cause of the error for you.
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If this is the case for you, the best thing to do, is simply wait for the servers to be fixed by the fine folks at Valve and to avoid buying anything until problems are resolved.
However, if you are experiencing errors and the servers are not down or experiencing any errors, it could be a problem with your Steam client. In this case, there are a few things you can always try, with the first being to simply relaunch your Steam app.
Usually, this will be enough to get the Steam app back into working order. You can also try restarting your PC or even reinstalling your Steam client.
Restarting your router or checking your Internet connection are also options, as they can sometimes fix any issues Steam has with connecting through your Internet.
Check out some of our other awesome Steam guides.