MacBook Pro A1502 Slow with fans running at high speed repair

Problem description
The machine running exceptionally slow with the fans spinning fast denotes a sensor issue. When a sensor is off, and it thinks the voltage/current is too high/low somewhere, or that the temperature is too high/low somewhere, it will clock the machine down and cause the fan to run at max speed. You have to use a tool that reads sensors like hwsensors, or boot into a Linux distribution to check your sensors, then chase down the circuit responsible for that sensor using the schematic/boardview. If all sensors are off, this usually points to an issue with the SMC or PP3V3_S5_AVREF_SMC.

Symptoms

 * MacBook running abnormally slow.
 * Fans spinning very fast.

Solution

 * Bad trackpad can cause sensor issue. Water gets to the trackpad first in many occasions.
 * Q5871 corroded, difficult to see without microscope, on edge of board.
 * U5870 corroded, on edge of board next to fan where water comes in.