Often this problem is the reason, if the tester starts with thebutton hold pressed, but switch off directly by releasing. The problem is enforced by a highcurrent background light for the LCD. The resistor R7 to the base of the PNP transistor T3 wasoptimized with the value 27kΩ too much to save power consuming. To improve the switchingwith lower battery voltage or lower current amplification factor of the PNP transistor T3, youshould reduce the resistance to 3.3kΩ.