A common fault is if your Xbox turns on, instantly flashing orange, with fan spin 100% (sometimes with turning off and repeating 3 times, if you also have a FRAG issue underlying this).

This is an early failure of the PIC chip and NVIDIA southbridge chip not liking something about the system. This can include several things including a bad PIC chip, NVIDIA chip, bad CPU/CPU, corrupt EEPROM or temperature sensors

This is very often the ADM1032 temperature sensor (an SO-8 package IC) near the clock capacitor often damaged from a leaking clock capacitor, so not communicating with the Southbridge Nvidia chip above it.

The temperature sensor receives 3.3V when powered on. The D+ and D- pins (temperature reading) have both an internal temperature sensor in the IC, and an external that goes to the CPU.

The D+ and D- lines have the capacitor C6F2 connected to them, and then go off to the CPU BGA pins.

The !ALERT and !THERM have no connections and no pull-up resistors so are always 0V.

The SCLK and SDAT go straight to the Nvidia Southbridge (MCPX X3).

If you are getting a flashing orange LED and full fan spin, and sometimes turning off after 3 surges, sometimes staying on, it is likely: