Proxy-related errors tend to repeat the same handful of codes. Knowing what each one actually means saves time compared to guessing.
407 Proxy Authentication Required
This means the proxy needs a username and password that either weren't provided or were entered incorrectly. Double-check credentials and confirm they haven't expired or been reset by the provider.
502 Bad Gateway
This usually means the proxy itself is reachable, but it failed to get a valid response from the destination server it was trying to reach on your behalf — often a sign the proxy server is misconfigured or the destination is blocking that proxy's IP specifically.
504 Gateway Timeout
The proxy didn't receive a response from the destination in time. This can mean the destination server is slow, or that the proxy's own connection to the wider internet is degraded.
Connection refused / timed out
This typically means the proxy server itself is offline or unreachable — extremely common with free proxies, and usually resolved simply by trying a different one rather than debugging further.