Xbox 360 0021

Check all caps are 0.1uF (6.3V, X5R), and continuity from them to the other side of the PCIe trace. Most boards have test points on the other side of each trace to verify this.
 * If you did not replace the GPU, skip this step.
 * PCIe Bus

Xenon: Check GPU to Southbridge Caps: C4D1, C4D2, C4D3, and C5D1. Check Southbridge to GPU Caps: C2C1, C2C2, C2C3, and C2C4.

HDMI Phat(Zephyr, Falcon, Jasper): Check GPU to Southbridge Caps: C4D1, C4D2, C4D3, and C4D7. Check Southbridge to GPU Caps: C2C1, C2C2, C2C3, and C2C4.

Trinity: Check GPU to Southbridge Caps: C5E1, C5E2, C5E3, and C5E4. Check Southbridge to GPU Caps: C3D2, C3E1, C3E2, and C3E3.

Corona: Check GPU to Southbridge Caps: C5E1, C5E2, C5E3, and C5E4. Check Southbridge to GPU Caps: C3D3, C3D4, C3D5, and C3D6.

Southbridge: If all else fails, replace Southbridge U2C1 (Phat) or U3D1 (Slim).

GPU: If all else fails, replace GPU U4D1 (Phat) or U5E1 (Slim).