Last updated: October 2026

To test continuity with a multimeter, turn the circuit off, set the dial to continuity (the speaker or sound-wave symbol), plug the black lead into COM and the red lead into the V/Ω jack, then touch the probes to both ends of the wire, trace or fuse. A beep means there is a low-resistance path; no beep means the path is open. Continuity and diode tests only work on unpowered parts, so power off first, every time.

This guide covers continuity mode, checking a cartridge fuse out of circuit, the diode test and what forward voltage numbers mean, and testing LEDs. It is written for low-voltage bench work: batteries, kits and boards.

Safety note

  • Power off and disconnect. Fluke’s continuity, fuse and diode procedures all start with the circuit de-energized and disconnected from any power source. Unplug the supply and remove batteries.
  • Discharge capacitors. Fluke warns that voltage may still be present because of charged capacitors and that they need to be discharged before a diode test. Our capacitor guide covers this.
  • Low voltage only. This guide does not cover household wiring, outlets or panels. Testing mains circuits is a job for a licensed electrician using CAT-rated gear and the meter maker’s manual.

Part 1: Continuity mode

What the meter is doing

In continuity mode the meter sends a small current through the probes and beeps when the resistance is low enough. The threshold varies by model. Fluke’s guide says a meter on a 400.0 Ω range typically beeps at 40 Ω or less. Fluke’s spec sheets for the 115 and 117 list the beeper as on below 20 Ω and off above 250 Ω. Check your own meter’s manual for its number.

Steps

  1. Turn off and disconnect the circuit.
  2. Insert the black lead into COM and the red lead into the VΩ jack.
  3. Turn the dial to continuity. On some meters continuity shares a dial position with resistance or diode test; your meter’s manual shows how to select it.
  4. Touch the probe tips together. The meter should beep and show a reading near zero. This proves the leads and meter work.
  5. Touch one probe to each end of the path you are checking.
  6. Beep: the path is continuous. No beep and OL on the display: the path is open.
  7. Turn the meter off when you are done to save the battery, as Fluke recommends.

Good uses for continuity

  • Checking a jumper wire or cable end to end.
  • Finding which header pin connects to which pad on a board.
  • Looking for a solder bridge between two neighboring pins after soldering. A beep between two pins that should not connect points to a short.
  • Checking a switch in both positions.

A beep across two points on a board does not always mean a direct wire. A low-value resistor or a forward-biased diode in the path can also trigger it. If the result surprises you, switch to resistance mode and read the actual value.

Part 2: Testing a cartridge fuse out of circuit

Small glass and ceramic cartridge fuses protect power supplies, amplifiers and test gear, including your multimeter’s current inputs. Test them out of the holder so the rest of the circuit cannot give a false reading.

  1. Power off and unplug the equipment.
  2. Remove the fuse from its holder.
  3. Set the meter to continuity, and touch the probes together to confirm a beep.
  4. Hold one probe firmly on each metal end cap of the fuse.
  5. Read the result. Fluke says that in continuity mode a beep means the fuse is good and no beep means it is blown. In resistance mode, a reading at or near 0 Ω means the fuse is intact, and OL (or a “1” on some meters) means it is open.

If a fuse is blown, replace it with the same current rating, voltage rating, speed and size printed on the cap or in the equipment manual. A blown fuse usually means something caused it, so find the fault before powering up again.

Your meter’s own fuses. Multimeters use fuses on their current inputs. Fluke describes the high-energy fuse in a meter as the part the user relies on for protection when a short happens during a current measurement. If your meter’s current ranges read zero, the internal fuse may be blown. Replace it only with the exact type the maker specifies. The multimeter spec table lists published fuse ratings for several popular meters.

Part 3: The diode test

What diode mode shows

A diode lets current flow one way. In diode mode the meter pushes a small current through the part and displays the voltage across it. Forward-biased, that number is the diode’s forward voltage drop. Reverse-biased, a good diode blocks current and the meter shows OL.

Your meter’s diode test voltage sets the limit on what it can measure. Fluke’s spec sheet for the 17B+ lists an open-circuit test voltage of 2.0 V with a short-circuit current below 0.6 mA. The 115 and 117 list a 2.00 V diode range. Other meters differ, so check yours.

Steps

  1. Make sure all power to the circuit is off and no voltage is present at the diode. Fluke lists this as the first step.
  2. Turn the dial to diode test (the diode symbol, a triangle pointing at a bar).
  3. Put the red probe on the anode and the black probe on the cathode. On small diodes the cathode end is usually marked with a band; the part’s datasheet confirms it.
  4. Read the display.
  5. Reverse the probes and read again.

Reading the results

Per Fluke’s diode guide:

What you seeWhat it means
About 0.5 to 0.8 V one way, OL the other wayGood silicon diode
OL in both directionsOpen diode
The same low reading (Fluke cites about 0.4 V) in both directionsShorted diode

For the most reliable result, lift one leg of the diode out of the circuit. Other parts connected in parallel can change the reading.

Part 4: Testing LEDs

An LED is a diode, so diode mode works on it, with one catch: its forward voltage is higher than a silicon diode’s.

  1. Find the leads. SparkFun’s LED tutorial says the anode (positive) is the longer leg; the other side is the cathode.
  2. Set the meter to diode test.
  3. Red probe on the anode, black probe on the cathode.
  4. A working LED often glows faintly and the meter shows its forward voltage. Reverse the probes and you should see OL.

Adafruit’s LED guide puts the forward voltage of most red, yellow, orange and light-green LEDs at roughly 1.85 to 2.5 V, with 2.2 V as a typical figure. Blue and white LEDs need more. If your meter’s diode test tops out near 2 V, as the Fluke 17B+ spec suggests, it may show OL on a perfectly good blue or white LED. In that case, test the LED with a bench supply and a series resistor instead. Our bench power supply guide explains current limiting.

Common mistakes

  • Testing with power on. Readings will be wrong and you can damage the meter. Power off first.
  • Skipping the probe-touch check. A broken lead looks exactly like an open fuse.
  • Testing in circuit and trusting a surprise. Parallel paths fool continuity and diode tests. Lift a leg and test again.
  • Leaving the red lead in the current jack. Return it to VΩ after any current measurement.

FAQ

What resistance counts as continuity? It depends on the meter. Fluke’s guide says a meter on a 400.0 Ω range typically beeps at 40 Ω or less, while its 115 and 117 spec sheets list the beeper on below 20 Ω. Your manual gives your meter’s threshold.

Can I test a fuse without taking it out? You can get a reading, but other parts in parallel can make a blown fuse look good. Fluke’s procedure removes the fuse first, and the circuit must be off either way.

What does OL mean in diode mode? Over limit: no current flowed. A good diode shows OL when reverse-biased. OL in both directions means the diode is open.

Why does my meter not light a blue LED? Its diode test voltage is probably below the LED’s forward voltage. Fluke lists 2.0 V for the 17B+. Use a current-limited supply and a resistor instead.

Is continuity the same as resistance mode? Continuity is resistance mode with a beeper and a threshold. For a real value, use the Ω setting. See how to use a multimeter for the basics.

Sources