Samsung rs21dpsm only a small green dash is iluminated on the control panel,on the door, aftre power failed, the question is, is the main pcb or the display pcb which is the likely to be damaged

try the main PCB. This has your CPu on it which controls this circuitry.
Try this web site, but use your COMPLETE model number as various sub models use different parts. https://www.encompassparts.com/model/SMGPN51F4500AFXZA/_/_/Samsung/PN51F4500AFXZA/51-INCH_PLASMA_4500_SERIES_TV_720P
main PCB: BN96-24574A
MAIN PCB BN96-19471A

http://www.samsungparts.com/Products/Parts_and_Accessories/PN51F4500AF/Default.aspx
MAIN PCB: BN94-06195C

E002
The main thermistor (TH1) has poor contact or an open circuit. The thermal
switch has an open circuit (TP1). The fixing heater has an open circuit. The
AC driver PCB is faulty. The DC controller PCB is faulty. The door switch
is faulty.

E002
0000 The temperature of the fixing film is as follows:
1. has exceeded 100°C, but does not reach 115°C within 1 sec thereafter.
2. has exceeded 140°C, but does not reach 150°C within 1 sec thereafter.
3. has exceeded 160°C, but does not reach 165°C within 1 sec thereafter.

Main cause: The fixing film unit is faulty (i.e., the main thermistor TH1 has poor contact,
or the fixing heater is faulty). The main power supply PCB is faulty.
The DC controller PCB is faulty.

Caution: To clear the error, execute the following in service mode:
COPIER>FUNCTION>FCLEAR>ERR.

Action See the description for E003.

E003
Action
1. State
Turn on the power switch, and clear E002/E003. Thereafter, turn off and
then on the power switch. Does the fixing heater operate?
NO: See "The fixing heater fails to operate."
2. Wiring
Is the wiring from the DC controller PCB to the fixing film unit normal?
NO: Correct the wiring.
3. Fixing film unit, DC controller PCB
Try replacing the fixing film unit. Is the problem corrected?
YES: End.
NO: Replace the DC controller PCB.

Main controller PCB
1) Turn offf and then on the power switch. Is the problem corrected?
YES: End.
NO: Replace the Main controller PCB.
Control panel PCB
2) Try replacing the control panel PCB. Is the problem corrected?
YES: End.

unplug your lvds cable going into the control pcb reconnect cable into the control pcb.if this does not work replace control pcb first,then also could be a defective panel.

Ok, you stated that you get a green for a second and then a couple of blinks and then, it turns off. This rules out the power board but, its main control partner is experiencing some trauma. Normally in the plasma TV set, the green is a go but, if the PCB(Power control board), doesn't agree, the the power board will shutdown the entire unit after giving a few blinks. Its very important to remember that, the power board and the power control board are two separate circuit boards. In your case, the PCB is down and it will need to be replaced.

Please rate and god bless.

that mean low power or The 24V output from the composite main power supply PCB is absent for 1 sec or more.
Main cause The wiring is faulty (short circuit, open circuit). The composite power supply PCB is faulty. The DC controller PCB is faulty.
Making Corrections
1. Connectors
Are the connectors J204 on the main power supply PCB and the connector J300 on the DC controller PCB connected securely?
NO: Connect the connectors securely.
2. Main power supply PCB Is 24 V supplied by the main power supply PCB (J204) to the DC controller
PCB (J300)?
(J204-5 <-> J300-5: OVU3, J204-6 <-> J300-6: OVU3,
J204-7 <-> J300-7: 24VU3, J204-8 <-> J300-8: 24VU3)

The main controller PCB has detected an error in the communication
between the DC controller PCB and the main controller PCB.

try to reconnect all conectors on your machine, check for toner dust on them, realese all wire holders, check the wires,...

It may be that the air vent is blocked by something as well but the filter is usually behind the air grills.

Thanks J

door lock blown this sometimes blows tracks off the main pcb behind the control panel. appliance doctor