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No USB / Dead Port Diagnosis

2 boards 7 diagnostic checks Updated March 15, 2026

Overview

Dead USB ports divide into two fault categories: power path (VBUS missing — device not powered) and data path (VBUS present but device not enumerated). Distinguishing these requires a USB VBUS measurement and a device enumeration test on each port. On Apple MacBooks, the USB-C controller (CD3215/CD3217) handles both USB PD negotiation and data mux — a dead CD3215 will fail both power and data simultaneously. On older USB-A ports, a load switch IC controls VBUS; data uses the USB hub controller further upstream.

Diagnostic Methodology

Follow these steps in sequence. Each step eliminates an entire fault zone — do not skip ahead.

1. Isolate port vs. upstream
Test same cable/device on all ports. If only one port dead: port-level fault (load switch, USB-C controller). If all ports dead: upstream hub or power rail fault.
2. Measure VBUS on dead port
USB-A: 5V between pin 1 (VBUS) and pin 4 (GND) with device plugged in. USB-C: 5V on CC1 or CC2 with cable attached. Absent VBUS = power path fault.
3. Check load switch enable
USB-A VBUS is controlled by a load switch IC (enabled by host controller). If EN signal present but VBUS absent: load switch failed (shorted/open MOSFET). Replace load switch.
4. USB-C: check CC pins
Probe CC1 and CC2 on USB-C port. Both should show 0V on source (host) side with no cable. A stuck-high CC pin = CD3215 fault — it can no longer correctly detect cable orientation.
5. Check USB hub controller
macOS/Windows USB device tree: if port appears in tree but device fails: data path fault (USB hub, cable). If port absent from tree: controller or power fault.
6. USB-C: inspect CD3215 under scope
CD3215 is BGA and susceptible to liquid damage on VCONN pins. Under 40× scope, look for corrosion or lifted balls. Reflow or replace if visible damage.

Per-Board Fault Trees

Board-specific checks ordered by failure likelihood. Most common root cause listed first.

MacBook Pro A1989 — Dead USB-C Start: PP5V_USB on dead port
⊞ Repair guide ◈ Schematic hub
  • CD3215 VCONN corrosion
    Most common after liquid damage. VCONN (5V) pin on CD3215 corrodes under liquid. No charging/data on affected port. Replace CD3215 — requires BGA rework station.
  • ISL9239 side fault
    If only charging is dead but data works: ISL9239 input side (VBUS path) fault. Check D7000 fuse and VBUS_IN on ISL9239 pin 1. Replace fuse first.
  • CC stuck high/low
    Measure CC1 and CC2 on dead port with DMM. If one reads unexpected voltage with no cable attached: CD3215 has failed internal FET. Replace CD3215.
  • PP5V_S0 absent
    Both USB-C ports dead, no power delivery at all: PP5V_S0 rail missing. See No Power diagnostic for S0 rail sequence.
MacBook Air A2337 — Dead USB-C Start: PP5V_USB on dead port
⊞ Repair guide ◈ Schematic hub
  • CD3215 failure (one side)
    A2337 has two USB-C ports with independent CD3215 controllers. Dead single port = replace that side's CD3215. Dead both ports = check ISL9239 or PP5V_USB rail.
  • No VCONN output
    VCONN powers the cable's e-marker chip. Without VCONN, Thunderbolt 3/4 and 100W USB PD will not negotiate. Measure VCONN (5V) on CC pins of cable.
  • USB3 data dead, USB2 works
    USB 3.x data goes through a retimer. If USB 2.0 works but USB 3.0/Thunderbolt fails: retimer or USB mux fault, not the CD3215 power path.

Frequently Asked Questions

How do I identify which CD3215 controller corresponds to which USB-C port?
The two USB-C controllers are mapped by PCB position relative to the ports. On A1989 and A2337: the left-hand ports (when viewing from bottom) are typically U6100, right-hand U6200. Check the schematic for your board number to confirm — the schematics hub at /schematics/ has per-board mapping. The CD3215 chip marking under magnification also identifies Left/Right via a small marker dot.
Can I test a USB-C port without specialized equipment?
Yes: (1) Try a known-good USB-C cable with a powered device. (2) Use a USB-C power meter (like Charger Doctor or UM25C) to confirm VBUS and CC negotiation. (3) In macOS, System Information → USB shows which controllers are enumerating — a missing controller tree indicates a power fault upstream of the controller.