Start with evidence
Run the tests that separate the likely branches
USB-C Display Path Readiness CheckCheck four documented prerequisites for sending video through USB-C, Thunderbolt, a dock, cable or adapter to a display.
Run →WebUSB Device TestRequest a compatible USB device through WebUSB and display the browser-exposed manufacturer, product, vendor, and product IDs.
Run →USB Vendor & Product ID CheckRead vendor and product IDs for a WebUSB-compatible device that you explicitly select.
Run →Charger Wattage MatchCompare a device's entered power requirement with a charger's rated wattage while keeping USB-PD/PPS and voltage-profile limits explicit.
Run →Decision table
What the observation changes
ObservationWhat it meansNext action
Mac charges but USB/display failPower delivery works; it does not prove USB4 or DisplayPort lanes.Swap to the dock's certified full-featured upstream cable.
USB devices work but display failsBasic USB data works; focus on video capability/topology.Test display direct to Mac, then verify dock display spec.
Dock works on one Mac port onlyPort/cable/physical state needs isolation.Repeat with same cable and inspect system/native diagnostics.
Sequence
Work from reversible checks to escalation
- 01
Use the dock's intended upstream cable.
- 02
Test dock on both Mac ports.
- 03
Separate charging, USB data and display observations.
- 04
Test key peripherals directly to Mac.
Likely areas
Keep these branches separate
Dock power deliveryCable not full-featuredDisplay pathUSB device/driverDock firmware
Repair boundary
When to stop changing settings
If both Thunderbolt ports fail with multiple known-good USB-C/Thunderbolt devices, run Apple Diagnostics/service path.