The common functional defects of LCD screens and LCD modules and their causes are as follows:
Main failures of LCD modules:
1. Dim display during testing
2. Garbled display when powered on
3. Missing COM (horizontal bars) or SEG (vertical bars)
4. No display
5. Program not running

Causes of functional defects in LCD modules:
1. Dim display: short circuit on PCB; short circuit due to dirty PCB; short circuit of zebra connector or electrodes; electrode misalignment during pressing.
2. Garbled display: faulty test fixture; damaged PCB chip; severe misalignment during pressing; short circuit between zebra connector and board.
3. Missing COM/SEG: open circuit on PCB; broken zebra connector electrodes during pressing; cracked electrodes during cleaning; cracked bonding area of zebra connector.
4. No display: faulty test fixture; abnormal power supply; poor contact of test clips; damaged PCB; severely cracked bonding area of zebra connector.
5. Program not running: issues with test fixture, power supply or test clips; damaged PCB.