So my Pi is giving me the red light of doom and I want to check to see if the power supply is faulty. How do I measure the voltage on the Pi2 from the power supply to ensure it is not faulty?
Pi2 Model B+ have test points on bottom of the board near the micro USB connector.
PP1 or PP2 is 5V from the power supply (Before the fuse) PP3 or any metal port shield is ground
From top of board you can test at GPIO pins Pin 2 or 4 is 5V (after the fuse) Ground - Use a port shield
You should see a voltage between 4.75 and 5.25 volts. Anything outside this range indicates that you have a problem with your power supply or your power cable.
Note: Even if the multimeter shows the correct voltage, you may have some power supply problems. A multimeter only displays the average voltage. If there are very short-lived dips or spikes in the voltage, these will not be shown by the multimeter. It is best to measure voltage when Pi is busy.
The red Power LED should never blink. It should be on continuously. If it blinks, or you see the little rainbow square on your screen, there is a "brown out", which is the voltage dropping below the 4.65v the Pi needs to operate. Consider getting a new power supply or swapping out the power cable for a shorter, fatter, better quality one.
See Troubleshooting power problems for more info