55LH90 (CHASSIS:LA92C) - LG TV Service Manual (repair manual). Page 46

LH40/LH55 Trouble shooting guide – HDMI

Check input connect JK500,JK501,JK502,JK503
Can you see the normal waveform?

Check DDC communication lines
(IC502)

JK500,JK501,JK502,JK503 may have problem. Replace this Jack.

Check the output of TDA9996HL(IC502)
_HDMI1,2,3 or HDMI4. . 
Can you see the normal waveform?

After checking the Power of this chip, you should decide to replace this or not.

Check HDCP communication lines(IC105)

After checking the Power of this chip, you should decide to replace this or not.

YES

YES

YES

NO

NO

NO

Video

NO

YES

YES

Check all power domain of LGE7329 (IC801)
(3.3V, 1.9V_CORE,1.26V) 

& crystal output (X800 / 12MHz)&FRC_RESET

Measure Power & waveform at L800,L801,L802,L803,L804,L806
because it’s more easy to check.
Can you see the normal waveform?

This board has big problem because FRC chip(LGE7329) have some troubles.
After checking thoroughly all path once again, You should decide to replace  LGE7329.
Although you replace it, you cannot see normal display, decide to replace Main chip(LGE3159)

This board has big problem because Main chip(LGE3159) have some troubles.
After checking thoroughly all path once again, You should decide to replace LGE3159 or not.