How to Test Micro USB Cable with Multimeter

How to Test Micro USB Cable with Multimeter

ByKen

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Here is how to test micro USB cable with multimeter on the bench. A micro-B plug has five contacts, not four, and the fifth one, the ID pin, tells you what kind of cable you hold. Check VBUS, D-, D+ and GND end to end, then read the ID pin: it should float on an ordinary cable and read close to zero ohms to ground on an OTG cable. Do all of it with the cable unplugged. This guide covers low-voltage micro USB cables only.

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.

Unplug both ends of the cable first. 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. 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 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

  • Check contacts 1, 2, 3 and 5 (VBUS, D-, D+ and GND) end to end in continuity mode, then check for shorts between them.
  • Read the ID pin, contact 4, to ground. A Micro-A plug ties it to ground, with a limit of 10 ohms. A Micro-B plug leaves it open or above 100 kilohms, which a later change notice raised to 1 megohm.
  • A short between D+ and D- inside the micro-B plug marks a charge-only cable, not an accident, according to one maker project.
  • Use resistance mode, not the beeper, to look at VBUS and GND, and trust only gross readings.

The five contacts

The Micro-USB specification, revision 1.01, numbers the plug contacts as in the table. The column of typical wire colors is typical only. Do not rely on color alone.

Contact Signal Typical wire Expected on a standard A-to-micro-B cable
1 VBUS Red Continuity to pin 1 of the Standard-A plug
2 D- White Continuity to pin 2 of the Standard-A plug
3 D+ Green Continuity to pin 3 of the Standard-A plug
4 ID Not applicable Open, or more than 100 kilohms to ground
5 GND Black Continuity to pin 4 of the Standard-A plug
Shell Shield Drain wire Continuity shell to shell

The specification allows these cables: Micro-A plug to Micro-B plug, Micro-A plug to Standard-A receptacle (an adapter), Micro-B plug to Standard-A plug, and a hardwired captive cable with a Micro-A plug. Cables must have a propagation delay of 10 ns or less and a physical length of no more than 2.0 meters. A change notice also lets a micro-USB cable twist the data and power lines together without a drain wire. A cable without a separate drain wire is therefore not defective for that reason.

Which cable do you hold?

The ID pin sorts the cables, and the specification states the rule in two parts. The ID pin on a Micro-A plug shall be connected to the GND pin. The ID pin on a Micro-B plug is not connected, or is connected to ground by a resistance above a limit. An on-the-go device decides which plug is inserted by checking whether the ID resistance to ground is below 10 ohms or above the limit.

Cable ID pin to GND Other points
Standard-A plug to Micro-B plug, the common data and charge cable Open (OL) or above 100 kilohms, or above 1 megohm after the change notice D+ and D- are separate wires
Micro-A plug to Micro-B plug, or Micro-A to Standard-A receptacle adapter Below 10 ohms (shorted to GND at the Micro-A plug) Silicon Chip calls these OTG cables, which short their GND and ID pins
Charge-only cable Open D+ and D- shorted in the micro-B plug, or missing
USB-C to micro-B (USB 2.0) cable Set by the applicable specification for the cable type 5.1 kilohms ±20% from CC (A5) to GND in the Type-C plug

The change notice matters if you read a number between the limits. It raised the minimum for a Micro-B plug’s ID resistance from 100 kilohms to 1 megohm, and says cables that have an ID resistance between 1 kilohm and 1 megohm are affected. A meter can read in that band. A reading of 50 kilohms is neither an open pin nor below 10 ohms, and the pages we opened say nothing about how a device should treat it.

What a multimeter can and cannot tell you

A meter finds opens, shorts and wires on the wrong contact. It cannot test data speed. A maker project on a micro-USB cable tester says its tester checks for power, for the D- wire, for D+ and D- shorted in the plug, for shell continuity and for ID tied to ground. The same page says its shield test does not prove there is a real shield, or that the shell is not simply shorted to GND. Silicon Chip’s cable tester adds that tests can run with only one end plugged in, mainly to ensure there are no shorts between any conductors. A multimeter does the same job one pair at a time. For a quick tool comparison, see our USB voltage checker guide.

Step 1: Set up and reach the contacts

Fluke says to put the black lead in COM and the red lead in the V jack, and to remove the 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. The micro-B contacts are small, and the pages we opened do not describe probing them directly. One way to reach them is a breakout board with a micro-B socket and pads, which is our suggestion, not one from the sources. Our voltmeter continuity test guide covers the beeper.

Step 2: Check the four main wires end to end

Touch one probe to a contact on the micro-B plug and the other to the matching pin on the far plug, one wire at a time: VBUS, D-, D+ and GND. A beep means a complete path, and if the circuit is open the meter will not beep, as Fluke says. Then check the shell from plug to plug. If only VBUS and GND beep, you have a power-only cable, which is not a fault. The maker project describes a power test as ensuring the VBUS and GND wires are present, and a data test as ensuring the D- wire is present.

Step 3: Read the ID pin

Set the meter to resistance. Put one probe on the ID contact and the other on the GND contact of the same plug. An open pin shows OL. For a Micro-A plug you expect less than 10 ohms. The Fluke 17B+ manual gives its 400.0 ohm range 0.1 ohm resolution and 0.5 percent of reading plus 3 digits, so a 4.0 ohm reading carries ±(0.005 × 4.0 + 3 × 0.1) = ±0.32 ohm, which is small next to the 10 ohm limit. Press the relative button with the probes touching to subtract lead resistance, which Fluke says is typically 0.2 to 0.5 ohm. For an open ID the same manual lists ranges up to 40.00 megohms, so a 1 megohm threshold is inside what the meter can read.

Step 4: Check for shorts between wires

Test every pair of the four wires at one plug, and the shell against each wire. Each pair should read OL. D+ against D- is the pair to treat with care. The maker project says a charger port signals up to 1.5 A by shorting D+ and D-, and that some cables short the data lines directly in the micro-B plug to save the cost of the data wires. It attributes the 1.5 A figure to the USB Battery Charging 1.2 specification, which we did not open, so treat the project’s explanation as the project’s. A beep between D+ and D- on a micro-B plug is therefore a clue that the cable is charge-only.

Step 5: Read VBUS and GND in ohms

Continuity mode on the Fluke 17B+ beeps below 70 ohms, so it passes a power wire that is badly poor. Use resistance mode on VBUS and GND, with the relative button set. The numbers to compare come from three places:

  • The USB 2.0 specification allows a maximum voltage drop for a detachable cable of 125 mV on VBUS and 125 mV on GND. A unit load is 100 mA and a high-power device draws up to five, so at 500 mA the largest resistance per wire is 0.125 ÷ 0.5 = 0.25 ohm (our arithmetic).
  • The Micro-USB specification says an adapter’s resistance through VBUS and GND, including contacts, shall not exceed 70 milliohms, and that low-level contact resistance is 30 milliohms maximum when measured at 20 mV maximum open circuit at 100 mA.
  • The same specification rates the VBUS and GND contacts at 1.8 A minimum, with 0.5 A on the other three.

The USB 2.0 specification lists maximum conductor resistance per 100 meters: 23.20 ohms for 28 AWG, 14.60 for 26 AWG and 9.09 for 24 AWG. Micro cables run up to 2.0 meters, so the table below applies those maxima (our arithmetic) at 0.5 A:

Wire gauge 0.5 m 1.0 m 1.5 m 2.0 m
28 AWG 0.116 ohm, 0.058 V 0.232 ohm, 0.116 V 0.348 ohm, 0.174 V 0.464 ohm, 0.232 V
26 AWG 0.073 ohm, 0.037 V 0.146 ohm, 0.073 V 0.219 ohm, 0.110 V 0.292 ohm, 0.146 V
24 AWG 0.045 ohm, 0.023 V 0.091 ohm, 0.045 V 0.136 ohm, 0.068 V 0.182 ohm, 0.091 V

The thinnest wire at the table maximum passes the 125 mV budget at 1.0 m (0.116 V) but fails it at 1.5 m (0.174 V). The budget is stated for 500 mA, and the pages we opened give none for higher currents. The meter cannot settle the question. Its 0.1 ohm resolution and ±0.3 ohm accuracy at these readings are as large as the wire. A reading of several ohms is far above every figure here and does show a bad cable. Calling readings near 1 ohm or above suspect is our own assumption. For direct milliohm readings, see our milliohm meter guide.

How to read the results

You see What it means What the sources leave open
OL on VBUS or GND end to end An open power wire. Silicon Chip says an open GND wire stops even charging Where it breaks
VBUS and GND beep, D+ and D- do not A power-only cable Whether the maker meant it
Beep between D+ and D- at the micro-B plug Data lines shorted, the charge-only design the maker project describes Whether the cable also has a data pair behind the short
ID to GND below 10 ohms A Micro-A plug or an OTG adapter Nothing further
ID to GND near 50 kilohms Neither an open pin nor below 10 ohms How a device treats a reading in that band

Mistakes to avoid

  • Testing a cable that is still plugged into a charger or computer.
  • Counting a beep as proof of a good power wire, when the beeper threshold is 70 ohms.
  • Skipping the ID pin and then wondering why an OTG adapter does not switch the phone to host mode. Silicon Chip says the grounded ID state tells a device to behave as a host.
  • Treating a missing drain wire as a fault when the change notice allows twisted-together quad construction.
  • Reading a short between D+ and D- as damage without checking whether the cable is a charge-only design.
  • Assuming a pass proves the cable carries data at speed.

How we chose these steps

We opened nine pages. Three are USB-IF documents: the Micro-USB specification with its change notices, the USB 2.0 specification and the USB Type-C specification. Two are independent builds or tests of cable testers: Silicon Chip and a maker project. Three cover meter use: Fluke’s continuity and resistance pages and the 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 lengths and tolerances 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 micro-USB charging cable measured with a handheld meter.
  • The pages we opened do not describe putting a standard meter lead on a micro-B plug contact.
  • The USB Battery Charging 1.2 specification was not opened, so the 1.5 A claim rests on the maker project alone.
  • No page says how phones treat an ID reading between the old and new limits.
  • The 125 mV budget is stated for 500 mA only.
  • The Fluke manual pages we opened do not state a CAT rating.

FAQ

What is the fifth pin on a micro USB plug?

It is the ID pin, contact 4. A Micro-A plug ties it to ground and a Micro-B plug leaves it open or at a high resistance. A device uses it to pick host or peripheral roles.

How can I tell if a micro USB cable is charge-only?

Check D+ and D- end to end. If neither beeps, the data wires are missing. If D+ and D- beep to each other at the micro-B plug, they are shorted there, which the maker project describes as a charging design.

Why does the cable pass continuity but still fail to charge?

The Fluke 17B+ beeper sounds below 70 ohms, far above the 0.25 ohm a power wire may have at 500 mA. A thin or damaged power wire can beep and still drop too much voltage.

What should the ID pin read on a normal cable?

OL, meaning open, or above 100 kilohms under the original specification and above 1 megohm under the change notice.

Can I test the cable while it is plugged in?

No. Fluke says to test a de-energized circuit, so unplug both ends first.

Bottom line

Prove the four main wires and the shell, read the ID pin to learn what the cable is, and let resistance mode catch only gross power-wire faults.

Sources

  • https://www.usb.org/sites/default/files/usb_20_20250603.zip (Micro-USB_1_01.pdf, MicroUSB ECN.pdf, Micro-USB ECN for Cable R3.pdf and 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://git.cuvoodoo.info/micro-usb_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)

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