Apple imac 20 inch early 2008 User Manual

iMac (20-inch Early 2008)  Take Apart — Chassis 2 21
Replace the hard drive fan.11 . 
Replace the h12.  ard driv e.
Replace the optical drive flex cable if removed.
13 .   

iMac (20-inch Early 2008)  Take Apart — Chassis 222
Replace the optical driv14. e.
Replace the logic board.
15 .  
Replace the right speaker16.  .
Replace the display panel.
17.  
Replace the front bezel.18.  
Clean and replace the glass panel19 . . Note: This step is very important. Follow the procedures 
to ensure the glass is clean and free of particles.
Replace the access door.
20.   

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 Service Source 
Troubleshooting
iMac (20-inch Early 2008) 

iMac (20-inch Early 2008) Troubleshooting — General Information 224
 
 
General Information
Serial Number
The serial number for this iMac model is located on the bottom of the computer stand. 

iMac (20-inch Early 2008) Troubleshooting — General Information 225
Power On Self Test (POST)
Intel-based Mac computers such as the iMac rely on a combination of tones and blinking LEDs to 
display Power On Self Test (POST ) error codes.If the computer detects out-of-specification or no SDRAM or the RAM installed does not 
• 
meet the appropriate specifications, the screen will remain black but the computer will beep  
This error condition may be due to physically damaged RAM, installing the incorrect...

iMac (20-inch Early 2008) Troubleshooting — General Information 226
How to Reset the System Management Controller (SMC)
The System Management Controller (SMC) is a chip on the logic board that controls all power 
functions for your computer. If your computer is experiencing any power issue, resetting the SMC 
may resolve it. The SMC controls several functions, including:Telling the computer when to turn on, turn off, sleep, wake, idle, and so forth.
• 
Handling system resets from various commands.•...

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Diagnostic LEDs
The iMac has four built-in diagnostic LEDs on the main logic board that can help you to 
troubleshoot the computer. The four LEDs are located to the right of the memory slot, under the 
hard drive data cable. Note: The cable has been removed in this photo.
LED #1 
Indicates that the trickle voltage from the power supply has been detected by the main • 
logic board. This LED will remain ON whenever the iMac is connected to...

iMac (20-inch Early 2008) Troubleshooting — General Information 228
Apple Hardware Test (AHT) Error Codes
The error codes listed below are the most likely codes that will be generated if a sensor cable is 
disconnected, dislodged, or if the system has a faulty cable or sensor. Clicking on the link in the 
chart will show you where the sensor and/or cable connector is located. Or, reference the next 
two logic board topics which show a picture of the logic board and the sensor locations.
AHT...

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AHT ErrorSuspected partSuggested ActionNotes
4SNS/1/40000000:TC0H
CPU temperature 
sensor (located on 
back side of MLB) is 
disconnected or faulty. Verify that the two 
pin sensor cable is 
securely connected to 
the MLB. The connector 
is located on the back 
side of the MLB and 
requires removal of 
the MLB to access it. If 
it continues to fail the 
diagnostic replace the 
logic board.If the sensor is 
disconnected 
the CPU fan 
will...

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Logic Board Sensor and Fan Connector Locations, Top Side
This is a photo of the top side of the iMac (20-inch Early 2008) logic board showing the sensor 
and fan connector locations.