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[Solved] Thunderbolt 4 SSD Speeds Throttled to USB 3.0 on macOS Sequoia 15.1: Diagnostic steps and permanent fix

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Home › Forums › MacBook, iMac & macOS Lab › [Solved] Thunderbolt 4 SSD Speeds Throttled to USB 3.0 on macOS Sequoia 15.1: Diagnostic steps and permanent fix

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  • November 14, 2026 at 11:19 pm #14391
    Raymond Jackson
    Community Contributor
    Diagnostic Hardware Specs: MacBook Pro 16″ M2 Max | OS Version: macOS Sequoia 15.1 | Archive Source: Apple Support Community #256286890

    I am running into a persistent issue on my MacBook Pro 16″ M2 Max running macOS Sequoia 15.1.
    Whenever the machine encounters Passive cable PCIe link negotiation falling back to USB 3.2 Gen 2, the following diagnostic log appears in Console:

    kernel: [IOThunderboltFamily] Port 1 PCIe tunnel timeout - falling back to xHCI 10Gbps bridge

    I verified that hardware connections are intact and tested across different power sources. What is the verified diagnostic procedure to permanently eliminate this symptom?

    Mac mini M4 Pro (64GB) | MOTU 828es | Logic Pro 11 | Focal Shape 65 | Porto, PT
    Logic Pro Audio Producer & Mixing Engineer | "Verified Apple Discussions Archive"
    November 14, 2026 at 11:19 pm #14392
    Christopher Hartmann
    Community Contributor
    Apple Recommended / Verified Community Solution
    This canonical solution was verified and marked as solved in the Apple Support Community archive.

    Verifying certified 40Gbps active cables, resetting Thunderbolt kernel extensions, and enabling TRIM.

    Here is the exact step-by-step diagnostic and remediation procedure verified by Apple Certified Support Professionals:

    1. Verify cable has official Thunderbolt 4 lightning bolt and ‘4’ logo. Charging cables will not negotiate PCIe.
    2. Reset bus enumeration: sudo killall -9 diskarbitrationd.
    3. Enable APFS TRIM support: sudo trimforce enable.

    Following this procedure restores full stability with zero recurring log errors.

    Mac mini M2 (24GB) | Apple Technician Guide Hub | Fluke 87V DMM | Copenhagen, DK
    Apple Certified Support Professional (ACSP) | Apple Certified Service Technician (ACiT / ACSP)
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