Full repair guide of LCD screen failure
LCD screen repair, bad LCD panel analysis, and solution
Working principle diagram of the LCD module

Definition
Vin: PWB input voltage (12V)
VDD: ASIC, source IC, gate IC driving power (3.3v)
VGH: TFT component switching voltage (~30V)
VGL: TFT component turn-off voltage (~ -6v)
VAA: step control voltage (~17V)
VCOM: liquid crystal reversal reference voltage (~7V)
LCD fault(Liquid crystal fault)
1. Picture color loss
Phenomenon: the picture is lack of color, or there are R/G/B color stripes under the step picture
Possible Causes | Repair proposal |
Note
|
Bad LVDS conn. | Replace LVDS connector | |
Missing/burned RX resistor | Replace RX resistor | |
ASIC NG
|
Replace ASIC
|

2. Striped screen
Possible Causes | How to Repair |
Note
|
ASIC NG
|
Replace ASIC
|
No EIO signal
|
COF
|
COF replacement (LCD bonding&pressing equipment required)
|
COF VAA short to GND |
COF damaged(S/G)
|
Check COF appearance first
|
|
COF comes loose | ||
PWM-IC NG
|
Replace PWM-IC
|
VAA short to GND |
Other components NG |
Components associated with VAA have less chance of NG
|
Analysis and solution:

3. Bad tone picture
Phenomenon: gradient fault occurs in the tone gradient screen or some gradient NG happens
Possible Causes | How to Repair | Note |
Gamma-IC NG
|
Replace Gamma-IC
|
|
PWB line scratch (gamma line) |
Insulation tape on the scratch | If the line is broken, the PWB must be replaced.(LCD bonding&pressing equipment required) |
COF-IC NG
|
COF replacement (LCD bonding&pressing equipment required) |
Gamma-IC NG
|
Analysis and solution:

4. Half of the picture is abnormal
Phenomenon: The left and right screens are different
Possible Causes | How to Repair | Note |
Skew conductive foam shorts RSDS | Reattach or remove the conductive foam | |
ASIC NG
|
Replace ASIC
|
RSDS abnormal
|
RSDS matching resistance NG | Replace RSDS resistor | |
S-COF-IC poor bonding on the PWB side | COF replacement (LCD bonding&pressing equipment required) | |
S-COF-IC NG
|
COF driver RSDS NG
|
Analysis and solution:
5. #If it is still NG, remove the COF from the middle or last COF, and the RSDS voltage returns to normal.

5. The panel color is whitish
Possible Causes | How to Repair | Note | |
Vcom value deviates | Vcom value is not adjusted | Reset VR knob | |
VR knob is broken | Replace VR knob | ||
VGH/VGL NG
|
PWM-IC NG
|
Replace PWM-IC
|
|
Gate-IC NG
|
Gate-IC replacement (LCD bonding&pressing equipment required)
|
||
Polarizer / CELL damaged |
To change the polarizer, a polarizer attaching machine is required
|
The degree of whitening of the picture changes with different viewing angles
|

Analysis and solution:
6. Slow screen switching

Possible Causes | How to Repair | Note |
WOA line is broken |
Replace PCB
|
PCB scrapped |
Gate-IC NG
|
Replace Gate-IC
|
|
X1 Poor Looseness |
Replace X1
|
Analysis and solution:

7. No display
Possible Causes | How to Repair | Note | |
LVDS connector NG
|
Wrong pin |
Replace conn.
|
|
Poor connection | |||
F1 fuse open
|
Replace FP1
|
||
Conductive foam short circuit element | Reattach or remove the conductive foam | Suitable for LCD module models with conductive foam | |
FFC Goldfinger short circuit |
Replace FFC
|
Suitable for LCD models with L/R boards | |
ASIC NG
|
3.3V short to ground |
Replace ASIC
|
|
No control signal | |||
DC/DC NG
(无VAA)
|
PWM-IC NG
|
Replace PWM-IC
|
|
X-COF NG
|
COF replacement (LCD bonding&pressing equipment required) | ||
QP1 burned | Replace QP1 | ||
G-COF NG
|
COF replacement (LCD bonding&pressing equipment required) | ||
Other components NG | Replace other components |

Analysis and solution:
Possible Causes | How to Repair | Note |
COF poor bonding (PWB side) | COF replacement (LCD bonding&pressing equipment required) | |
Poor COF |
Analysis and solution:
Backlight failure
Possible Causes | How to Repair | Note |
Lamp line is broken | Replace the lamp tubing | Depending on the backlight structure, there will be different results. The failure of the performance may be a point-off, or it may be a backlight with a dark band. |
Broken high and low voltage plugs |
Replace plugs
|
|
Lamp holder | Replace lamp/lamp holder | |
Poor engagement of lamp holder | Replace lamp/lamp holder | |
Lamp is broken | Replace the lamp | |
Lamp welding failure | Resolder | |
Analysis and solution:
Possible Causes | How to Repair | Note |
Lamp line is broken | Reconnect / replace lamp tubing | Depending on the protection status of the power board, it may be a backlight with a dark band or it may be a point-off. |
Plug disconnected / burned | Replace plugs | |
Lamp is broken | Replace the lamp | |
Lamp welding failure | Replace the lamp | |
Possible Causes | How to Repair | Note |
The light bar is short-circuited to ground | Insulate or change the light bar | |
Lightbar plug pin off | Change the light bar | |
The light bar cable plug is not fully inserted into the conn. | Reinsert the light bar | |
Lightbar conn. Pin is skewed | Change conn. Or re-insert the pin | |
Light strip conn. Off | Reweld Conn. | If the PCB copper foil comes off, the light bar needs to be replaced |
Bad light bar | Replace the light bar |
how about a 7 inch car display that is half black. is there a problem in the circuit or replace the lcd screen
We think you need to check the flexible PCB of the LCD, if there is corrosion then the PCB needs to be repaired or replaced;
if the PCB is in good condition then we recommend just replacing the whole LCD.
Magnifique ce article
Thanks!
Thanks vеry interesting blog!
How about an LCD that is all white. No picture at all. Good article.
1. Check whether the power supply has DC voltage output.
2. Check whether there is a DC voltage output corresponding to the LCD glass at the screen line end of the LCD driver board
3. check whether the logic board or screen integrated board fuse has voltage.
4. check whether the screen 5 voltage is normal.
Can you repair electronic fault on LCD PCB for LG model LP156WF3-(SL)(B3) ?
yes
this article is a big help to understand how lcd panel works and fails,thank you.
You are welcome. We are glad it can help you:-)