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모델 A1419 / EMC 3070 / Mid 2017 / 3.4, 3.5 또는 3.8 GHz Core i5 또는 4.2 GHz Core i7 Kaby Lake Processor (ID iMac18,3) / Retina 5K Display. 시스템이 매우 유사하므로 이전 iMac Intel 27" Retina 5K Display (Late 2014 & 2015) 안내서를 참고하십시오.

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Diagnostic lights and no screen issue

Hello everyone,

I have a bit of an issue.

I’m experienced technician, but this was first 2017 5k 27 inch imac I was repairing at the component level.

Problem was that there was no image output. Tried external usb-c, no chime, no logo, but fans work.

I had previously repaired earlier 2015 imac with same fault, but found failed EFI , so I fixed it by replacing EFI chip. So I done it this time also. Bought a preprogrammed EFI chip for this particular logic board, soldered it on, tested connection - soldered great with solid connection.

Powered on - and issue is still here.

Then I noticed that only first two diagnostic lights light up. Now, does this mean that actual GPU might be fried? I know older imacs and macbooks had these issues, but new ones?

I work in university as technician and they will not approve the entire logic board replacement, and quite frankly I was saving this iMac from getting binned.

Any ideas? Suggestions?

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@pegasus_y so this is what this all looks like during startup.

Diagnostic LEDs (A)

LED 1

Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will turn on when you connect the iMac to a working AC power source. The LED will remain on as long as the computer is on or asleep.

When the computer has been shut down correctly, LED 1 behavior may differ:

If a startup event is scheduled in System Preferences/Energy Saver, then LED 1 will stay on after a correct shutdown. If no startup event is scheduled in System Preferences/Energy Saver, then LED 1 will turn off and will stay off as long as the power cord is kept connected and an AC power source is present. Disconnecting the power cord and plugging it back in will turn this LED back on, even if the computer is still off.

After disconnecting and reconnecting the AC power source, this LED could remain off:

If the AC power source is missing or disconnected.

If the logic board is disconnected from the power supply or the AC receptacle.

If the power supply board is faulty.

LED 2

Indicates that the computer is turned on. This LED will be on as long as the computer is turned on (but is not asleep) and the power supply and voltage regulators are working correctly.

LED 3

Indicates that the logic board and GPU are communicating. This LED will be on when the CPU is communicating properly with the GPU. If LEDs 1 and 2 are on and LED 3 is off, then the backup battery (on the back of the logic board) may need to be reseated or the logic board may need replacement.

LED 4

Indicates that the logic board and LCD panel are communicating. This LED will be on when the computer is turned on and a video signal is being generated. If LED 4 is on and there is no image on the display, then the LCD panel or the cables between the LCD and logic board might be installed incorrectly or need replacement.

LED 5

Indicates the logic board and LCD panel are communicating. This LED is on when the computer is turned on, a video signal is being generated, and the LCD Panel is signaling to turn on the backlight. If LED 5 is ON and there is no image on the display, then the LCD backlight or circuitry on the logic board may be malfunctioning. The LCD panel or the cables or the logic board may need replacement.

LED Startup Sequence

LED 1 = Power is available.

If no LED is visible:

Disconnect the power cord from the computer and wait 15 seconds to reset the power supply and LED status. Reconnect the power cord and check the LED status again.

Verify the AC source.

Verify that a known-good power cord is connected.

Verify the cable connection between the AC inlet and the power supply.

Verify the cable connection between the power supply and the logic board.

Verify the power supply.

LED 1 + LED 2 = Power is available and the system is turned on.

If the second LED is not visible when the power button is pressed:

Verify that the power button is connected to the power supply.

Verify power button functionality.

Verify the cable connection between the power supply and the logic board.

Verify the power supply.

Verify the logic board.

LED 1 + LED 2 + LED 3 = Power is available, the system is turned on, and the GPU was found.

If the third LED is not visible after the system is turned on:

Verify whether the fan is running when turned on (reset SMC and PRAM, verify backup battery voltage for proper startup).

If the fan is not heard, go to the “No Startup” troubleshooting flow.

LED 1 + LED 2 + LED 3 + LED 4 = Power is available, the system is turned on, the logic board is communicating with the GPU, and the internal LCD was found.

If the fourth LED is not visible after the system is turned on:

Verify the internal DisplayPort cable (eDP) connections between the LCD panel and the logic board.

Inspect the LCD display cables for cable damage.

Verify external video functionality and proceed according to the result:

If an external display works, verify/replace the LCD panel.

If an external display does not work, verify/replace the logic board.

LED 1 + LED 2 + LED 3 + LED 4 + LED 5 = Power is available, system is turned on, GPU is working, logic board is communicating with the LCD panel, and the LCD panel has turned on the LCD backlight.

If the fifth LED is not visible after the computer is turned on:

Verify the internal DisplayPort cable and backlight cable connections between the LCD panel and the logic board.

Inspect the LCD display cables for cable damage.

Using a flashlight, check the following areas on the panel:

Look in the top left corner of the panel to see if there is a dim Apple logo (the Apple menu logo).

Look in the center of the panel to see if there is a login screen.

Look at the bottom of the panel to see if there is a Dock visible.

If the following items are visible, there may be something wrong with the display cables or the logic board.

Proceed to the next step.

Replace the LCD panel, check that all the LEDs are visible, and that the backlight functions. If the computer does not function after replacing the LCD panel, replace the logic board.

Diagnostic LEDs and Test Pads

Diagnostic LEDs and Test Pads for iMac (Retina 5K, 27-inch, 2017)

Warning: HIGH VOLTAGE. Be extremely careful when working inside the computer while power is applied and the system is energized. Avoid touching the logic board or power supply while the computer is plugged in. Be very careful not to touch tools to logic board components other than the test pads.

After unplugging the computer from the electrical outlet, wait two minutes before removing the display panel, disconnecting modules, or substituting cables and components. This will allow the power supply and logic board time to discharge.

Refer to the following articles for more information:

TP833: iMac and Displays: Power Supply Cover Instructions

TP820: iMac (27-inch): Safety

The iMac (Retina 5K, 27-inch, 2017) model has five diagnostic LEDs and two pairs of test pads that can help you troubleshoot the computer without removing the logic board.

The coin battery, located on the back of the logic board, provides power to the real-time clock (RTC) and parameter RAM (PRAM) when the computer is not connected to an AC power source. The RTC maintains the date and time, while the PRAM stores information such as speaker volume, screen resolution, startup disk selection, and recent kernel panics. The coin battery is designed to last several years and does not normally require replacement. However, if there are issues with the functions listed above, the RTC and PRAM may need to be reset or the coin battery may need to be replaced.

A. Diagnostic LEDs

See the full description of LED behaviors below the locator image.

B. Coin Battery Voltage Test Pads

Shut down and unplug the iMac. Allow approximately two minutes for the power supply to discharge.

Measure the coin battery voltage by using a voltmeter set for DC. Place the probes on the pads. TP1901 = – (negative, same as chassis ground), TP1900 = + (positive) in location B, shown below. If the voltage is 2.7 volts DC or less, then the coin battery should be replaced.

C. Real-Time Clock (RTC) Reset Pads

Shut down and unplug the iMac. Allow approximately two minutes for the power supply to discharge.

Reset the RTC by shorting the pads in location C, shown below. Use the tip of a flat-blade screwdriver to touch both pads at the same time.

Caution: Do not make contact with any of the surrounding components or traces while performing this procedure. Some of the smaller components can be broken off very easily. Physically damaged boards warrant a logic board replacement.

I am sure you did the whole resetting procedure as well but just in case

Quick Check Procedures

System Configuration for Macs with the Apple T2 Security Chip

Important: For Macs with the Apple T2 Security Chip, the repair process is not complete for certain parts replacements until the AST 2 System Configuration suite has been run. Failure to perform this step will result in an inoperative system and an incomplete repair.

TP1657: System Configuration for Macs with the Apple T2 Security Chip

For MacBook Pro (2018): Display assembly, logic board, top case, and Touch ID board

For MacBook Air (Retina, 13-inch, 2018): Logic board and Touch ID board

For iMac Pro: Logic board and flash storage

For Mac mini (2018): Logic board

Resetting the System Management Controller (SMC)

The System Management Controller (SMC) is a chip on the logic board that controls all power functions. On some Mac computers, the Apple T2 Security Chip integrates several controllers—such as the SMC, image signal processor, audio controller, and SSD controller. If the computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, or fan noise issues, resetting the SMC may resolve it.

For more information and instructions to reset the SMC on different computer models, refer to HT201295: How to reset the System Management Controller (SMC) on your Mac.

Note for iMac: If the power button is pressed while the power cord is being inserted, the iMac will enter a mode that runs the fans at full speed. For more information, refer to HT204463: iMac: Fans run at full speed after computer turns on.

Note for iMac Pro (2017): If the power button is pressed while the power cord is being inserted, the iMac will enter Device Firmware Upgrade (DFU) mode and will need to be restored.

Resetting Nonvolatile RAM (NVRAM)

NVRAM stores certain system and device settings in a location that macOS can access quickly. Exactly which settings are stored in the computer’s NVRAM varies depending on the type of computer, connected devices, and drives. To reset NVRAM:

For information, refer to HT204063: How to Reset NVRAM or PRAM on your Mac.

@@pegasus_y post some good pictures of your logic board with your QUESTION. These iMacs love to pop some inductors which "could" be causing those issues. Adding images to an existing question

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It may, but it’s also a bit more complex than that. There are several systems that have to be in place to get this LED to light, even beyond the GPU being up and running. The GPU might be bunk, but it’s also possible something else is causing the issue. I’m not overly familiar with iMac boards, they are a bit different than MacBooks. But just a quick look at this schematic, there’s actually a lot going on electronically between LED 2 being on and LED 3.

I don’t have access to schematics for this specific board (I was using the 2015 model for reference), but the first thing I would do is just give the board a once over, or twice over and see if anything looks weird, burned up, blown, etc.

I would hazard to say that iMac boards are more likely to just have a simple short due to a blown cap, or a single component sort of failure, since they are MUCH less likely to have been liquid damaged. Or dropped.

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