Apple imac 21 5 inch late 2009 User Manual

iMac (21.5-inch, Late 2009) — General Troubleshooting 212010-11-18
Diagnostics
Run Apple Service Diagnostic version 3S135 to determine if any thermal sensors or blowers 
are malfunctioning. If ASD is not available, run Apple Hardware Test version 3A189 by holding 
down the D key during startup. When a test reports an error, reseat appropriate connections. 
If issue persists, replace the corresponding part (sensor, blower, logic board, video card, or 
power supply). See Apple Hardware Error code chart...

iMac (21.5-inch, Late 2009) — General Troubleshooting 222010-11-18
AHT Error codeSuspected part and 
Location Suggested Action
Notes
4SNS/1/40000000: 
TC0H CPU temp sensor 
(located on back side 
of logic board).  Note: 
this sensor is not 
available as a separate 
service part Check CPU sensor cable 
connection to logic board. 
It requires removal of the 
logic board.
If the sensor is 
disconnected, the 
CPU fan will run 
at full speed.
4SNS/1/40000000: 
TG0H GPU temp sensor 
(located on back side 
of...

iMac (21.5-inch, Late 2009) — General Troubleshooting 232010-11-18
Diagnostic LEDs
Depending on iMac model, there are either three or four built-in diagnostic LEDs on the logic 
board that can help troubleshoot the computer. LEDs are located on bottom right edge of logic 
board (under a piece of black mylar tape) and can only be seen when looking through lower 
vents of rear housing.    

iMac (21.5-inch, Late 2009) — General Troubleshooting 242010-11-18
LED Functions
LED #1 
• Indicates that trickle voltage from power supply has been detected by logic board. This LED 
will remain ON whenever the iMac is connected to a working AC power source. The LED will 
remain on even when computer has been shut down or put to sleep. The LED will turn off 
only if AC power source is disconnected or power supply is faulty.
LED #2
•  Indicates that logic board has detected power from power supply when...

iMac (21.5-inch, Late 2009) — General Troubleshooting 252010-11-18
LED Startup Sequence
LED #1 = Power available.  
If no LED is visible:• Verify AC source 
•  Verify known-good AC cable is connected
•  Verify cable connection between AC inlet and power supply
•  Verify cable connection between power supply and logic board
•  Verify power supply
LED #1 + LED #2 = Power available, and system is powered on. 
If second LED is not visible when power button is pressed: • Verify power button connection to...

iMac (21.5-inch, Late 2009) — General Troubleshooting 262010-11-18
Logic Board Test Points
Test points, which can be used to verify proper power flow, are accessible on logic board when 
LCD panel is removed. All voltages given in Troubleshooting Symptom Charts assume that 
computer is plugged into a known-good power outlet with a known-good AC cable.  Some 
guidelines for using test points: 
• Warning: HIGH VOLTAGE:  Use extreme caution when live testing!
•  Do NOT lean over or touch the power supply...

iMac (21.5-inch, Late 2009) — General Troubleshooting 272010-11-18
Sensor and Fan Connector Locations
Ambient temp sensors and fans connectors locations are shown below. Ensure cables are 
correctly routed and the sensors and fans are properly connected. If a sensor or fan is faulty or 
not connected, Apple Hardware Test and Apple Service Diagnostic will generate an error code.  

iMac (21.5-inch, Late 2009) — General Troubleshooting 282010-11-18
Block Diagram
Refer to this diagram to see how modules are interrelated.
SO-DIMM
Antenna
Antenna
AntennaSO-DIMM
Temperature 
SensorTemperature 
Sensor
Fan
MLB
SO-DIMM
Display
AC/DC
Power 
Button
Bluetooth
Speakers
Hard DriveOptical Drive
AC Inlet
T emperature 
SensorCamera
Microphone
Speakers
Backlight
Controller
Fan
Fan
MXM
802.1 1n
Ambient
T emperature 
Sensor
IR Sensor
SD Card Reader
SO-DIMM  

iMac (21.5-inch, Late 2009) Symptom Charts — Startup and Power 29 2010-11-18
Symptom Charts
Follow steps in the order indicated below. If an action resolves the issue, retest system to verify.
Note: A compilation of Quick Check tables is available at:
http://service.info.apple.com/QRS/en/quickreference.pdf
Startup and Power
No Power, Dead Unit
Unlikely cause: speakers
Quick Check
SymptomsQuick Check
No Power, Dead Unit
•  No power
•  No fan spin
•  No startup chime
•  No image on external display
•  No...

iMac (21.5-inch, Late 2009) Symptom Charts — Startup and Power 30 2010-11-18
3.  Press power button while 
monitoring diagnostic LEDs. 
Verify if LED #2 turns ON and 
remain ON after pressing 
the power button. You may 
alternately check for a power 
request signal between pin 12 
and pin 1 of logic board. YesPower supply functioning and 
logic board detects supply 
voltages. Go to Won’t Start 
Up symptom flow.
No Go to step 4.
4.  Disconnect AC cable then 
remove power supply and 
carefully inspect...