One of the most frustrating things Apple ever did was lock out Target Display Mode on every single 27-inch 5K Retina iMac (Late 2014 through 2020).
The 5120×2880 LG display inside a 2014–2020 5K iMac is virtually identical to the panel inside Apple’s $1,599 Studio Display. Yet if you buy a new M1, M2, M3, or M4 MacBook Pro for work, Apple’s official support page tells you that you cannot plug it into your 5K iMac to use as a monitor.
Fortunately, our bench technicians use three proven workarounds to turn any 2014–2020 iMac into an external monitor for a modern MacBook—ranging from a 100% free software + cable trick to a permanent true 5K hardware conversion.
Comparison: The 3 Ways to Use an Old iMac as a Monitor
| Method | Cost | Resolution & Refresh | Lag / Latency | Best For… |
|---|---|---|---|---|
| 1. The Wired OpenCore “AirPlay to Mac” Trick | $0 (Free) | Up to 4K / Retina Scaled | Very Low (over USB cable) | Reading docs, coding, spreadsheets, second screen |
| 2. Hardware Dongle (Luna Display / Duet) | $40–$129 | Retina Scaled | Low | Users who don’t want to patch macOS on the iMac |
| 3. DIY 5K LCD Driver Board Conversion | $125–$180 | True Native 5K (5120×2880 @ 60Hz) | Zero (Pure Hardware) | Video editors, designers, or iMacs with dead logic boards |
⚡ Method 1: The Free Wired OpenCore “AirPlay to Mac” Trick
When Apple released macOS Monterey (and later Ventura and Sonoma), they introduced AirPlay to Mac, which lets one Mac extend its desktop onto another Mac’s screen.
Apple artificially restricted this feature to 2019+ iMacs—but OpenCore Legacy Patcher (OCLP) unlocks it on any 2012–2017 iMac! Even better, you don’t have to rely on laggy Wi-Fi: macOS automatically routes AirPlay video over a direct USB or Thunderbolt cable if the two Macs are plugged into each other.
Step-by-Step Setup:
- Upgrade the Old iMac Using OpenCore:
- Follow our EZ No-USB OpenCore Guide to update your 2012–2017 iMac to macOS Ventura or Sonoma.
- Open the OpenCore-Patcher app on the iMac → click Settings → check
Allow AirPlay to Mac(enabled by default in modern OCLP builds under SMBIOS spoofing) → click Build and Install OpenCore to your internal drive and reboot.
- Turn On AirPlay Receiver on the iMac:
- On the iMac, open System Settings → General → AirDrop & Handoff.
- Toggle AirPlay Receiver to ON.
- Set Allow AirPlay for: to Everyone (or Current User if signed into the same Apple ID).
- Connect the Two Macs with a Cable (The Zero-Lag Secret):
- Connect a USB-C data cable (or USB-C to USB-A 3.0 data cable on 2014–2015 iMacs) between your new MacBook and the back of the iMac.
- macOS automatically establishes a high-speed IP bridge over the physical cable so video doesn’t stutter over Wi-Fi!
- Extend Your Desktop:
- On your new M1/M2/M3/M4 MacBook, open Control Center → Screen Mirroring (or System Settings → Displays →
+Add Display). - Select your iMac as an Extended Display!
- On your new M1/M2/M3/M4 MacBook, open Control Center → Screen Mirroring (or System Settings → Displays →
Method 2: The Hardware Dongle Method (Luna Display)
If your old iMac is still running an older OS like High Sierra, Mojave, or Catalina and you don’t want to update it with OpenCore:
- Plug a Luna Display USB-C or Mini DisplayPort hardware dongle into your new MacBook.
- Install the free Luna Display app on both the MacBook (Primary) and the old iMac (Secondary).
- Connect the two machines together via an Ethernet cable or Thunderbolt bridge cable for smooth desktop extension.
Method 3: The True 5K Studio Display Hardware Conversion (DIY Driver Board)
If your 2014–2020 27-inch 5K iMac has a dead motherboard, failed GPU, or you want 100% uncompressed, zero-latency 5120×2880 @ 60Hz color accuracy for photo and video editing, you can convert the iMac chassis into a true standalone monitor.
[!WARNING] High-Voltage Power Supply & Glass Precaution: Opening a 27-inch iMac exposes the internal AC/DC power supply board, which contains high-voltage capacitors that can retain a dangerous electrical charge even after unplugging the power cord. Wear safety glasses when cutting display adhesive and never touch exposed power supply capacitors.
How the Conversion Works:
- Check the LCD Panel Model Number:
- Unseal the 27-inch 5K display glass using an iMac adhesive cutting wheel.
- Look at the sticker on the back of the LG panel:
- 2014–2015 5K iMacs: Usually
LM270QQ1 (SD)(A2)or(B1). - 2017–2020 5K iMacs: Usually
LM270QQ1 (SD)(C1),(D1), or(F1)(supports wider P3 color gamut).
- 2014–2015 5K iMacs: Usually
- Order the Matching 5K Driver Board (
R1811orJRY-W9RQUHD):- Purchase a dedicated 5K eDP controller board matched to your exact
(SD)(xx)panel revision.
- Purchase a dedicated 5K eDP controller board matched to your exact
- Gut the Old Logic Board & Install the Controller:
- Remove the old Apple motherboard, fans, and hard drive (reducing the weight and heat of the iMac by half!).
- Connect the iMac’s original eDP display ribbon and backlight cable into the new 5K driver board.
- Route the USB-C (DisplayPort Alt-Mode + 65W Power Delivery) or HDMI/DisplayPort cable out through the rear circular power cord hole.
- Reseal the Screen: Apply fresh 27-inch VHB display adhesive strips. You now have a silent, zero-lag 27-inch 5K monitor for under $160!
Need Help Upgrading or Repurposing Your 27-inch iMac in Chicago?
Whether you want us to configure OpenCore Legacy Patcher so you can use wired AirPlay to Mac, upgrade your slow iMac with an internal SSD, or recover data from a dead iMac logic board, our technicians at Arbor Mac can help.
Visit our shop in downtown Chicago at 8 S Michigan Ave, Suite 312 (Willoughby Tower across from Millennium Park) daily from 12pm–10pm, or calculate your Mac service quote online in 10 seconds.