When the PD Handshake Fails on the Line: A Practical Debug Checklist
The light's on, voltage's there, but the phone charges at 5V and takes forever. The fix isn't a new charger — it's finding where the PD conversation broke. Here's the step-by-step I use on the bench.
A failed PD handshake is the most common "it tested fine but the customer says no" fault on a charger line. The symptom is always the same: power is present, yet the device trickle-charges or drops and reconnects. The problem isn't the hardware — it's one step of the negotiation that never completed.
The checklist below runs in order, power first, protocol last. Each step is independently verifiable, so by the end you've narrowed the fault to one of three buckets: the cable, the device-under-test, or the tester setup.
1Confirm VBUS is present
Measure VBUS at the Type-C receptacle with a multimeter or scope (A4/B9 / A9/B4 pairs). No 5V? That's a power-path problem, not a protocol problem — stop here and check the supply and rectification before touching PD.
2Check the CC line and the cable
Type-C has CC1 and CC2; plug orientation decides which one carries the handshake, and the cable's e-Marker sets the current ceiling. Swap in a known-good cable and try both orientations. A large share of "handshake fails" tickets disappear at this step — the cable was the fault, not the charger.
3Confirm the roles
The charger is the Source; the phone (or your tester acting as the sink) is the Sink. A tester must emulate a Sink to draw out the charger's capabilities. If your bench setup is wired as two sources, nothing will ever negotiate — and that reads identically to a "dead" charger.
4Watch the PD message sequence
A full PD negotiation runs: Source_Capabilities → Request → Accept → PS_RDY. If your protocol tester supports PD communication capture (message logging), read the trace and see which message never arrives. No capture tool? Power up in stages and watch the VBUS step changes on a scope — the jump from 5V to 9V/15V/20V tells you the negotiation got at least that far.
5Test the private protocols separately
Huawei SCP/FCP, Samsung AFC, OPPO VOOC/SuperVOOC and others don't ride standard PD. A unit that passes PD can still fail private-protocol charging. Verify each with a tester that covers the full 20+ protocol list — don't assume a green PD result means the charger is fully good.
6Run it hot before you sign off
Thermal throttling drops power under load — a unit that negotiates fine cold can fall back to 5V after a few minutes. Run a full-load aging pass with ripple and thermal monitoring so a heat-driven droop doesn't surface at the customer's desk instead of your bench.
Most handshake failures land in one of three places: a cable you can swap, a real defect in the device-under-test's firmware or hardware, or a tester setup error (sink/source reversed, private protocol not enabled). Walk the list top to bottom and the location shows up fast.