How to Test a USB Cable with a Multimeter

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Here is how to test a USB cable with a multimeter. Unplug it from everything, set the meter to continuity, and check that each pin at one plug reaches the matching pin at the other plug and that no two wires touch. Then read the power wires in ohms. A meter finds opens, shorts and crossed wires. It cannot show data speed, and its continuity beeper can pass a cable whose power wires are too poor to charge well. This guide covers USB-A, USB-B and USB-C cables on the bench.

Electrical safety first

OSHA’s guide to controlling electrical hazards says an electric shock can range from a slight tingling sensation to immediate cardiac arrest. Work on household mains circuits can kill, and nothing below asks you to touch an outlet, a panel or service wiring. That work belongs to a licensed electrician, and OSHA says only qualified electricians trained in safe lockout procedures should maintain electrical equipment.

Test the cable with both ends unplugged from the computer, the wall adapter and the device. Fluke’s continuity guide says to test with the circuit de-energized and to isolate the part from other components. The USB 2.0 specification rates a cable at 30 V rms maximum, which is not a mains rating, so never use a USB cable on anything else.

The meter’s CAT rating must suit the circuit. Fluke says to choose a tool rated for the highest category you could potentially use it in, and it says never to use a damaged meter or damaged test probes. The Fluke manual pages we opened do not state a CAT rating, so this guide rates no meter and never calls one safe for household mains.

Quick answer

• Use continuity mode on one wire at a time: VBUS to VBUS, GND to GND, D+ to D+ and D- to D-, then check every pair of wires for a short. Expect OL between wires.
• Do not trust the beeper for power wires. The Fluke 17B+ manual says it sounds below 70 ohms, while a USB 2.0 power wire may have at most 0.25 ohm at 500 mA.
• Read VBUS and GND in resistance mode, use the relative button to cancel lead resistance, and treat only gross readings as proof of a bad cable.
• On a USB-A to USB-C cable, the specification calls for about 56 kilohms between CC and VBUS inside the C plug.

What each wire should connect

USB 2.0 plugs number four contacts: 1 VBUS, 2 D-, 3 D+ and 4 GND, with the shield on the shell. The specification lists red, white, green and black as typical wiring, which is typical only. Do not rely on color alone.

Wire	USB-A or USB-B plug	USB-C plug on an A-to-C cable	USB-C to USB-C cable (USB 2.0 type)
VBUS	Pin 1	A4, B4, A9, B9	A4, B4, A9, B9 at both ends
D-	Pin 2	A7	A7 at both ends
D+	Pin 3	A6	A6 at both ends
GND	Pin 4	A1, B1, A12, B12	A1, B1, A12, B12 at both ends
CC	None	A5, joined to VBUS through 56 kilohms ±5%	A5 to A5, one wire
VCONN	None	Not connected	B5 to B5, only in electronically marked cables
Shield	Shell	Shell	Shell

The specification says all VBUS pins are connected together inside the Type-C plug, and the same for ground, so a reading on any one VBUS pin stands for all four. It also says shield and GND are connected within the plugs on both ends of an A-to-C cable. Silicon Chip also says it is common to find GND shorted to the shield on some USB-C cables without causing problems, so do not read shield-to-GND continuity on a Type-C cable as a fault.

What a multimeter can and cannot tell you

A meter can find an open wire, two wires touching and a wire landing on the wrong pin. Total Phase says a cable may have electrical connections present but still be wired incorrectly if a conductor is connected to the wrong pin, and it separates pin continuity (the connection exists) from pin mapping (it reaches the correct pin). Check both. A meter cannot test high-speed data. Silicon Chip notes that its cable tester does not test high-speed signal integrity either.

Power is the hard part. The same article says a dedicated tester runs pulses of up to 1 A through VBUS and GND and reports the resistance and voltage drop, because that is how it finds cables that cause intermittent faults. The Fluke manual pages we opened do not state the current its resistance mode uses, so a reading tells you about the wire’s resistance and not about how a charging cable behaves under load. For a related tool, see our low-resistance tester guide.

Step 1: Set up the meter

Fluke says to put the black lead in COM and the red lead in the V jack, and to remove leads in reverse order, red first. Turn the dial to continuity, which often shares a position with resistance. With the probes apart the display may show OL, and with the probes touching it should read close to zero. Look over the plugs first for bent contacts or cracked housings. Our continuity checker guide explains the beeper in more detail.

Step 2: Check each wire end to end

Touch one probe to a contact on the first plug and the other to the same signal’s contact on the far plug. A beep means a complete path. Fluke adds that if the circuit is open, the meter will not beep. Work down the table: VBUS, GND, D+, D-, then the shield. Fluke’s guide says to use the lowest range setting for parts that should have low resistance, such as electrical connections.

On a cable that has no D+ and D- wires, only VBUS and GND will beep. That is a charge-only cable, and it is not a fault. Silicon Chip’s tester reports whether a cable is suitable for power only, USB 2.0 or USB 3.2, and it says an open GND wire means the cable will not function, even for charging.

Step 3: Check for shorts between wires

Put one probe on VBUS and the other on each remaining wire in turn: GND, D+ and D- at the same plug. Then put one probe on D+ and the other on D-, and repeat for GND against D+ and D-. Each pair should read OL. A beep or a low reading means two wires touch. That is six pairs for a four-wire cable. On a Type-C plug the shield and GND may legitimately connect.

Step 4: Read the power wires in ohms

Continuity mode is a pass or fail at one threshold. The Fluke 17B+ manual says the beeper sounds continuously below 70 ohms, so a power wire at 69 ohms would still beep, yet the USB 2.0 detachable cable budget allows only 125 mV on VBUS. The specification defines a unit load as 100 mA and a high-power device as up to five unit loads, so at 500 mA the largest resistance per wire is 0.125 V ÷ 0.5 A = 0.25 ohm. That is our arithmetic, and 69 ÷ 0.25 = 276 times that budget.

Switch to resistance mode and read VBUS and GND separately. The table converts the limits in the two specifications into resistances.

Rule	Voltage budget	Current assumed	Largest resistance per wire (our arithmetic)
USB 2.0 detachable cable, VBUS	125 mV	0.5 A	0.125 ÷ 0.5 = 0.25 ohm
USB 2.0 cable, GND	125 mV	0.5 A	0.125 ÷ 0.5 = 0.25 ohm
Type-C cable rated 3 A, VBUS	500 mV	3 A	0.5 ÷ 3 = 0.167 ohm
Type-C cable rated 3 A, GND	250 mV	3 A	0.25 ÷ 3 = 0.083 ohm
Type-C cable rated 5 A, VBUS	500 mV	5 A	0.5 ÷ 5 = 0.100 ohm
Type-C cable rated 5 A, GND	250 mV	5 A	0.25 ÷ 5 = 0.050 ohm

The Type-C specification says the IR drop includes the contact resistance of the mated plugs and receptacles at each end, so the wire alone gets less than these figures. The USB 2.0 specification gives maximum conductor resistance per 100 meters by wire gauge. Here is that table turned into lengths, again our arithmetic:

Wire gauge (AWG)	Maximum ohms per 100 m	1 m cable	2 m cable	Longest length within 125 mV at 0.5 A
28	23.20	0.232 ohm, 0.116 V	0.464 ohm, 0.232 V	1.08 m
26	14.60	0.146 ohm, 0.073 V	0.292 ohm, 0.146 V	1.71 m
24	9.09	0.091 ohm, 0.045 V	0.182 ohm, 0.091 V	2.75 m
22	5.74	0.057 ohm, 0.029 V	0.115 ohm, 0.057 V	4.36 m
20	3.58	0.036 ohm, 0.018 V	0.072 ohm, 0.036 V	6.98 m

Read the table as a warning about thin wire. The specification says the minimum acceptable gauge is calculated assuming the attached device is high power, and a 28 AWG power pair at the table maximum would drop 0.232 V on VBUS alone over 2 m at 0.5 A. The specification also says the resistance of the two conductors of any pair must not differ by more than 5 percent, and that the plug-shell-to-plug-shell DC resistance must be less than 0.6 ohm. The shell figure is within what a handheld meter can read.

Now compare with the meter. The Fluke 17B+ manual gives its 400.0 ohm range a resolution of 0.1 ohm and an accuracy of 0.5 percent of reading plus 3 digits. At a 0.25 ohm reading that is ±(0.005 × 0.25 + 3 × 0.1) = ±0.301 ohm, which is larger than the reading. Fluke says test leads add typically 0.2 to 0.5 ohm and the relative mode subtracts them. Press relative with the probes touching, then measure the wire. The result still cannot separate a 0.23 ohm wire from a 0.5 ohm one. It can expose a wire at several ohms, which sits far above every limit in the tables. Treating readings near 1 ohm or more as suspect is our own assumption, not a figure from the specifications. See also our milliohm meter guide.

Step 5: Check the CC resistor on a USB-A to USB-C cable

This applies only to a cable with a USB-A plug on one end and a Type-C plug on the other. The specification says pin A5 (CC) of the Type-C plug is connected to VBUS through a 56 kilohm resistor, ±5 percent, which is 53.2 to 58.8 kilohms. Set the meter to ohms, touch one probe to A5 and the other to a VBUS pin. On the 17B+ manual’s 400.0 kilohm range, resolution is 0.1 kilohm and accuracy 0.5 percent plus 2 digits, so a 56.0 kilohm reading carries ±0.48 kilohm and a good part should show about 52.7 to 59.3 kilohms. OL means the resistor is missing or open. A USB-C to USB-C cable has no such resistor, and its CC wire should simply have continuity from A5 to A5.

How to read the results

You see	What it means	What the sources leave open
OL end to end on VBUS or GND	An open power wire. Silicon Chip says an open GND wire stops even charging	Where along the cable it breaks
OL end to end on D+ or D- only	A charge-only cable, or a broken data wire	Which of the two applies
A beep between two different wires	A short between them	The cause
VBUS reaches a different pin than expected	A mapping error, which Total Phase says can exist even when connections are present	Whether the cable is repairable
Power-wire reading of several ohms	Far above the 0.25 ohm and Type-C budgets	An exact pass or fail value for a meter

Mistakes to avoid

• Testing a cable that is still plugged into a charger or computer. Fluke says to test only de-energized circuits.
• Passing a cable because it beeps. The 17B+ manual says its beeper sounds below 70 ohms.
• Trusting wire colors. The specification calls them typical.
• Skipping the short checks. A cable can show continuity on every wire and still have two wires touching.
• Calling shield-to-GND continuity a fault on a Type-C cable.
• Assuming a pass proves data works. Neither a meter nor Silicon Chip’s tester checks high-speed signals.

How we chose these steps

We opened nine pages. Two are USB-IF specifications: the USB 2.0 specification package and the USB Type-C specification. Two describe cable testing from the tester side: Silicon Chip and Total Phase. Three cover meter use: Fluke’s continuity and resistance pages and the Fluke 17B+ manual. Two cover safety: Fluke’s multimeter safety guide and OSHA’s electrical hazards booklet. We left out forum posts, retailer pages and any page that blocked our request. The tables of resistances and lengths are our own arithmetic from the stated limits.

What we could not verify: what the sources do not say

• No page gives a pass or fail resistance for a charging cable measured with a handheld meter.
• The pages we opened do not describe putting a standard meter lead on a Type-C plug contact.
• The USB 2.0 specification gives the voltage budget and the conductor table separately, and the pages we opened do not give a gauge-versus-length chart.
• The 0.5 A figure is the high-power limit of five unit loads, not a measured charging current.
• Silicon Chip’s page prints its good-cable benchmark with a garbled unit, so we did not use it.
• No page covers USB 3.x SuperSpeed wires beyond noting that a tester can detect them.
• The Fluke manual pages we opened do not state a CAT rating.

FAQ

Can I test a USB cable while it is plugged in?

No. Fluke says continuity testing is done on a de-energized circuit, and a plugged-in cable may be connected to a wall adapter. Unplug both ends first.

What should a good USB cable read in ohms?

The pages we opened give limits, not a typical reading. The USB 2.0 budget works out to 0.25 ohm per power wire at 500 mA, and Type-C budgets are tighter at higher current ratings. A handheld meter cannot resolve that closely.

Why does my cable beep but not charge my device?

The Fluke 17B+ manual says its beeper sounds below 70 ohms, far above what a power wire may have. A wire can also land on the wrong pin, which Total Phase says is separate from having a connection.

How do I know if a cable is charge-only?

Check D+ and D- end to end. If only VBUS and GND beep, the cable carries power but no data.

Bottom line

Use continuity to prove that every wire reaches the right pin and that none touch, then use resistance mode only to catch gross power-wire faults, and trust a load test for anything finer.

Sources

• https://www.usb.org/sites/default/files/usb_20_20250603.zip (usb_20.pdf inside; opened 2026-10-21)
• https://www.usb.org/sites/default/files/USB%20Type-C%20Spec%20R2.0%20-%20August%202019.pdf (opened 2026-10-21)
• https://www.siliconchip.com.au/Issue/PE/2022/December/USB+CABLE+TESTER (opened 2026-10-22)
• https://totalphase.com/blog/2018/08/continuity-testing-usb-type-c-cables-using-advanced-cable-tester (opened 2026-10-22)
• https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-for-continuity (opened 2026-10-21)
• https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-measure-resistance (opened 2026-10-21)
• https://media.fluke.com/f8ca9de4-481a-4125-8a46-b2d9006f431f_original%20file.pdf (opened 2026-10-21)
• https://www.fluke.com/en-us/learn/blog/safety/multimeter-guide (opened 2026-10-21)
• https://www.osha.gov/sites/default/files/publications/OSHA3075.pdf (opened 2026-10-21)