Macs are a thing of beauty. It has been said often that Apple garners just as much attention for the usability of their products as they do for their industrial aesthetics. Very few computer manufacturers worldwide compete on Apple's level of industrial engineering, however the true test of any computer device is how well it functions.
Regardless of which device you use to accomplish your work, they all have one thing in common: It's painstakingly annoying when they're not working at 100% correctly.
Macs are no different, which is why Apple has an easy-to-use diagnostic utility built into each computer called Apple Hardware Test (AHT) manufactured before June, 2013. For those devices manufactured after June, 2013, the newer, much more simple-to-understand Apple Diagnostics (AD) utility will be used.
AHT, while light-weight, is ghastly at relaying important troubleshooting information the system has detected during a scan. Often times, no errors are found— which is a good thing— meaning the hardware checks out as operating properly. Though when errors are indeed found, the amalgamation of letters and numbers defy all logic, and worst of all, leave no room for even the slightest hint as to what is wrong with the system. This requires providing the information directly to Apple or an authorized service center for a more lay explanation.
While having an authorized professional inform you of the particular issue(s) affecting your Mac computer and providing solutions to correct them are not a bad thing at all, depending on your skill level as a repair technician and availability of replacement parts, many hardware issues affecting Apple's computers could be repaired within minutes or hours as opposed to days or weeks, if the device must be shipped out to a repair depot.
A word of caution though— while knowing the cause of the problem is all well and good, it is a far leap to actually possessing the skills necessary to undertake the repair yourself. Especially when it comes to non-end-user-serviceable parts, such as Logic Boards, Power Supply, or LCD panels, just to name a few. Risk of further damage to your computer — and even your health — is a very real threat when not working using the proper tools and safety precautions.
With that said, I present the breakdown of AHT codes and what they mean as a way to better interpret the issues affecting your Apple computers. This information should allow you to best determine the course of corrective action necessary to repair a device.
As an example, we'll use the code "4MOT/4/40000002:HHD-0" to explain how the code works.
The first four characters in the code identify the component that has reported a failure. While a cursory glance would point to an issue affecting "HDD-0," this assumption would be incorrect if you glance at the list below of known component codes.
- 4ETH: Ethernet controller
- 4IRP: Main Logic board
- 4MLB: Logic board controller
- 4PRC: Processor
- 4HDD: Hard disk
- 4MHD: External disk
- 4YDC: Video card
- 4SNS: System sensor
- 4MOT: Fan motor
- 4MEM: Memory module
- 4AIR: AirPort wireless card
Comparing the "4MOT" error code against the list above, it reveals that the failure actually stems from a fan motor, likely the cooling fan responsible for keeping HDD-0 running cool and efficiently.
Most error codes are pretty straightforward to decode except "4SNS" which indicates a system sensor error. These are difficult to place because there are many sensors in use on a given Mac so a secondary table is used to conclude which sensor is faulty.
This time, let's use the example code "4SNS/1/40000000: TH00-22.214.171.124" to explain how the sensor codes work.
When a sensor error is detected, the code "4SNS" is used to identify that it is a sensor, however the second half of the error code will provide the necessary information to determine what type of sensor it is. I denotes it is an electrical sensor, V indicates a voltage sensor and lastly T signifies it to be a temperature sensor.
The location of the component that the sensor looks after can be gleaned by reviewing the list below for the matching code, paying special attention to case-sensitivity.
- A: Ambient Air Sensor
- B: Battery
- C: Central Processors (CPU)
- D: DC (Direct Current)
- e: PCI-Express Slot
- F: FireWire Port
- G: Graphics Processor (GPU)
- H: Hard Disk
- h: Heat Sink (or Heat Pipe)
- L: LCD display
- M: Memory (or Memory Riser Boards)
- m: Miscellaneous (Peripherals such as a Battery Charger)
- N: North Bridge (Logic Board Controller)
- O: Optical Drives
- P: Power Bus
- p: Power Supply
- s: Trackpad
- W: Airport Wi-Fi Card
In comparing the code "4SNS/1/40000000: TH00-126.96.36.199" to the lists, we have determined that the "4SNS" error identified a sensor being faulty and that the "T" error code indicates it to be a temperature sensor in the Hard Disk as the "H" error code aligns to that component.
With this information at your fingertips, making sense of the jumble of letters and numbers helps to correctly inform the repairing technician of where the problem lies and how to go about repairing it.
Luckily for us, Apple has made Apple Diagnostics much more layman friendly as the error codes are followed with a plaintext explanation of what was found to further aid in the repair process.
Did I miss something? Or Would you like to share an AHT experience? Let us know in the comments section please.
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Jesus Vigo is a Network Administrator by day and owner of Mac|Jesus, LLC, specializing in Mac and Windows integration and providing solutions to small- and medium-size businesses. He brings 19 years of experience and multiple certifications from several vendors, including Apple and CompTIA.