How can I test my APU with voltmeter
Just check for shorted capacitors around or under the apu.
On what programme I have to put my voltmeter
Continuity mode and put black probe on ground and red probe on each side of capacitor. If both sides of capacitor beeps, it means there shorted.
For continuity mode it will look like a weird upside-down horse shoe thing. (otherwise know as the omega symbol)