Apple imac 20 inch early 2008 User Manual

iMac (20-inch Early 2008) Troubleshooting — General Information 2 31
Logic Board Sensor Locations, Back Side
This is a photo of the back side of the iMac (20-inch Early 2008) logic board showing the GPU and 
CPU sensor connector locations.
Cleaning Tools for the LCD and Glass
The following tools are necessary when servicing this product. Order a cleaning kit, (076-1277), 
which contains all the items listed, or purchase the tools individually.Suction cups, one pair (922-8252)
• 
Sticky silicone roller...

iMac (20-inch Early 2008) Troubleshooting — General Information 232
LensPen Cleaning Tool
LENSPEN products are simple to use and are effective at removing smudges and fingerprints on 
the inside/outside of the glass panel. The two LENSPEN products that have been tested on the 
iMac are shown below, the VidiMax and the LapTop-Pro. Refer to www.lenspen.com for more 
information. 

iMac (20-inch Early 2008) Troubleshooting — General Information 233
Ionizing Air Gun
The ionizing air gun is an optional tool you may want to consider when cleaning iMac glass 
panels. The ionizing gun performs electrostatic elimination and dust removal instantly with ion 
balanced air.  Various sources can be found on Google. 

iMac (20-inch Early 2008)  — Symptom Charts  234
Symptom Charts 
When to Use the Symptom Charts
Before turning to the symptom charts, you should have completed the following steps on the 
Apple General Troubleshooting Flowchart:
Gather information
1. 
Verify the problem2.  
Try quick fixes3. 
You consult the symptom charts as part of the Research troubleshooting step (and sometimes as 
part of the Try Quick Fixes step).
How to Use the Symptom Charts
The Symptom Charts included in this chapter will help...

iMac (20-inch Early 2008)  — Symptom Charts  235
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 is located.
AHT ErrorSuspected partSuggested ActionNotes
4SNS/1/40000000:Tp0P
Power supply sensor 
pin/power supply Verify that the single 
pin connector on the 
top edge...

iMac (20-inch Early 2008)  — Symptom Charts  236
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 run at full...

iMac (20-inch Early 2008)  — Symptom Charts  237
Power Issues
System Intermittently Shuts Down or Restarts Unexpectedly
Check the connection of the power cord on both ends. Verify that the power cord is securely 1. 
plugged into a good AC outlet and that the cable is fully inserted into the back of the 
computer. Did re-seating the power cord resolve the problem? 
 
Yes: The power cord may have a loose fit or it wasn’t fully inserted into the receptacle on the 
back of the iMac. Replace the AC power cord...

iMac (20-inch Early 2008)  — Symptom Charts  238
Does the computer continue to unexpectedly shut down or restart? 2. 
 
Yes: Replace the DC, Power Supply/SATA/Inverter cable (922-8188) shown below. Note: The 
latest version of the DC Power Supply/SATA/Inverter cable be identified by the presence of 
a part number label on the end of the connector that plugs into the main logic board. With 
earlier versions of the cable, the part number label is located on the cable, but the label is 
hidden by the...

iMac (20-inch Early 2008) 
 — Symptom Charts 
 239
No Power
The computer will not turn on. The display remains black and there are no sounds from the fans 
or drives.
1. Verify the power outlet is good. Plug a different device into the socket to ensure there is power, 
or plug the iMac into another outlet. Does the iMac power on now?
Ye s: Resolved. Bad outlet.
No : Go on to the next step.
2. Check the power cord. Use a known good power cord. Does the iMac power on  now? Ye s: Your power cord has failed....

iMac (20-inch Early 2008)  — Symptom Charts  240
LED # 2 is On: The Power Supply is functioning. Go on to the next step.
LED # 2 comes on momentarily or stays Off: Replace the Power Supply.
9. At this point in the Power On process, you should hear a boot chime and see that LED #1 and 
#2 are On. Do you hear a boot chime? Ye s: The power systems of the computer are working correctly. See “Troubleshooting No 
Video” in this chapter.
No : The logic board is not passing the Power-On Self Test (POST )....