I have measured my dual diode with diode check from DMM and the first reading of first diode revealed 0.4V and 0.8V on the second diode. It is clearly the voltage drop between two diodes are different. How can I tell which one of them is good or bad?
MM
I have attached a datasheet for you to understand more.
Are you sure if you are measuring different voltages across pins 1 and 2 and across pins 1 and 3. It seems more than coincidence that the diodes in this device are in series and you measure twice the voltage across one than the other.
Maybe your 0.4V is across one diode and the 0.8V is across two.