1021 Xbox 360 secondary error code | Three red rings guide
Learning about your issue
I'll start with the basics here. The 1021 secondary error code deals with the GPU chip on the motherboard. This is one of many infamous errors that go along with a typical RROD. This issue is not something tackled easily as permanent repairs need to be done by a professional with the right equipment. There are, however, some things you can do to buy time without that. So let's discuss it.
Using an X-clamp fix
An X-clamp fix can be used to address this problem, maybe. I myself only suggest this kind of fix as a temporary measure as it can fix your problem for a few days, or a few years. There are special kits to replace the X-clamps on your GPU heatsink to (hopefully) fix your 1021 secondary error code. If you want a more permanent solution then your Xbox 360's GPU would need to be reflowed or replaced. That process requires special equipment and is not possible for the average individual.
