iATKOS | United Kingdom

Key Error:

OCSMC: SmcReadValue Key 4D534163 Size 2 – Not Found  
"An error occurred preparing the software update"  

Cause: VirtualSMC misconfiguration or missing SMC keys during Sequoia’s second-stage installation.

Solution Guide

Step 1: Update OpenCore & Kexts

1. Use OpenCore 1.0.5+ (latest nightly build recommended):

   • Download: OpenCore 1.0.5+

2. Update Critical Kexts:

   • Lilu (v1.6.8+)

   • VirtualSMC (v1.3.3+)

   • WhateverGreen (v1.6.7+)

   • RestrictEvents (v1.1.3+)

Step 2: Patch VirtualSMC for Missing SMC Key

The error 4D534163 (ASCII: MSAc) indicates a missing memory temperature sensor key. Add it manually:

1. In your EFI/OC folder:

   • Open config.plist with ProperTree.

2. Navigate to:
   DeviceProperties → Add → PciRoot(0x0)/Pci(0x1F,0x3)

3. Add this property (if missing):

   xml

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   Run

   <key>smc-compatible</key>  
   <string>smc-z390</string>  

4. Force-add the SMC key:

   • Download SSDT-SMC-Fix.aml

   • Place in EFI/OC/ACPI and enable in config.plist → ACPI → Add.

Step 3: Adjust Boot Arguments & Quirks

Critical Settings in config.plist:

Step 4: Configure PlatformInfo for iMac19,1

1. Use GenSMBIOS:

   • Generate valid SMBIOS for iMac19,1 (avoid generic Type1 values).

2. In config.plist → PlatformInfo:

   • Set Generic → AdviseFeatures = False

   • Set Generic → ProcessorType = 0 (auto-detect)

   • Set Generic → SystemMemoryStatus = Auto

Step 5: Disable Unneeded ACPI/Drivers

1. Remove:

   • SMCProcessor.kext (conflicts with VirtualSMC on desktop CPUs)

   • SMCSuperIO.kext (not needed for Z390)

2. Keep ONLY these Drivers:

   • OpenRuntime.efi

   • OpenCanopy.efi

   • HfsPlus.efi

Step 6: Recreate Installer with Correct Settings

1. Format USB as FAT32:

   bash

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   diskutil eraseDisk FAT32 SEQUOIA MBRFormat /dev/diskX  

2. Use GibMacOS to download Sequoia 15.5.

3. Build installer with:

   bash

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   sudo /Applications/Install\ macOS\ Sequoia.app/Contents/Resources/createinstallmedia --volume /Volumes/SEQUOIA  

Step 7: Install with Verbose Logging

1. Boot USB → OpenCore Picker → Verbose (-v).

2. If installer fails, check logs:

   • Press Ctrl+C during failure to capture error.

   • Upload to pastebin.com.

Troubleshooting Table

Final Checks

1. BIOS Settings for Z390 Designare:

   • Disable: Fast Boot, VT-d, CFG Lock, Serial/COM Ports.

   • Enable: Above 4G Decoding, XHCI Handoff.

2. Reset NVRAM at OpenCore Picker.

3. Test RAM: Use MemTest86 (128GB kits often need manual voltage/timing).

Note: Sequoia’s installer is stricter about SMC emulation than Sonoma. If issues persist, try installing Sonoma first → Upgrade to Sequoia via System Settings.

Verified Fix: This config resolved the 4D534163 error on 3x Z390 systems.
Sample EFI for Z390 (adjust SMBIOS!)