Hi Paul,
Thank you for writing in and your detailed description.
Could you please send us some images of your wiring to let us check the problem details?
With your help, we can find a solution more quickly.
And here is the wiring diagram of the product, please check it:

Hope this can help you.
Best regards

Hello Paul,
The third setup mode is used for setting voltage alarm threshold.
When the measured voltage is over the measured range, the backlight and the voltage will flash to remind you.
Instruction of the Key button:

1.Backlight control
Short press the key to turn on or off the backlight, the backlight has memory function, it can store the on or off state when power off.
2.Reset energy
Step1: Long press the key until the power display area display “CLr”, then release the key.
Step2: At this moment, the energy is flashing to prompt this is the energy reset state, if short press the key again, then the energy value is cleared and exit the reset flashing state.
Step 3: If there is no operation within 5 seconds, it means the energy value is not cleared and will exit the reset state.
3.Set voltage alarm threshold
Step1: Long press the key until the power display area display “SET”, then release the key.
Step2: After enter this state, the voltage area display the current high voltage alarm value, the current area display the current low voltage alarm value and the last digit begin to flash, then you can short press to plus 1, when there is no operation over 3 seconds, it will switch the digit position automatically, from the high voltage alarm value to the low voltage alarm value, there are total six digits, the range of the voltage alarm threshold is 6.5～99.9V;
Step3: After finish the setting, long press the key until the screen display “PASS” ,that means you set successfully and will exit the setting state automatically.

Hope this can help you.
Best regards