Temp Error while on power supply

Hi

I have this problem since last printing went wrong :
(Printer stopped in the middle of the printing : Extruder was stuck in the pla object)

No when I connect Repetier-Host
It shows correct temp (around 20°C) for the extruder

But if I switch on the power supply, temperature drops to 0°C
And I got the MIN TEMP triggered and the M999 sentence

I am afraid !

What do you think it may be ?

Try to switch the temp around and se if you get the same value. Rule out if the temp is defect or something else

Most likely your thermistor is at fault, have you tried a new one?
When you power from computer you are only powering the mega if i'm not wrong? And once you switch to power supply you are powering from the ramps board, so although I doubt it, it might be a grounding issue in the ramps board? Sorry, totally guessing here.