0021 Xbox 360 secondary error code | Three red rings guide
An issue of human error
The 0021 secondary error code on the Xbox 360 is a complicated one that usually revolves around human error. This error is likely caused by someone replacing the GPU with an improper model that doesn't match. This would throw the 0021 error code. There are quite a few other much more complicated issues that can cause this. These include checking signals on resistors associated with the GPU. Due to the complex nature of those checks I will not go into great detail here. For the vast majority of people, this error would require you to replace your motherboard entirely to fix the problem. Otherwise, You would need to remove the GPU and replace it with one that matches what originally shipped with your motherboard to clear this 0021 secondary error code.
