A resistor works by limiting electrical current, and you can measure whether it's doing that job with a multimeter

A resistor is a component that slows down electrical current. When you need to know if a resistor is working, you measure its resistance value using a multimeter — a handheld tool that reads electrical properties. The process takes about 30 seconds per resistor and tells you whether the component is good, damaged, or completely failed.

The most common reason to check a resistor is troubleshooting: a circuit isn't working the way it should, and you want to rule out a bad component before replacing it. You might also check resistors when you're learning electronics and want to verify that the color bands on the component match its actual resistance value.

Key Takeaways

  • Set your multimeter to the ohms (Ω) setting, starting with a range higher than the resistor's rated value.
  • Touch one probe to each end of the resistor — the order does not matter, and the resistor does not need to be powered.
  • A reading close to the resistor's color-band value means it is working; a reading of zero or infinity means it has failed.
  • Remove the resistor from the circuit before testing it, because other components in the circuit will interfere with the reading.
  • If your first range gives a reading of zero or infinity, try a lower ohms range to get a more precise measurement.

Set your multimeter to measure resistance

Turn the dial on your multimeter to the ohms setting, marked with the Greek letter Ω. Most multimeters have several ohms ranges — typically 200Ω, 2kΩ, 20kΩ, 200kΩ, and 2MΩ. Start by selecting a range that is higher than the resistor's rated value. If you are checking a 470Ω resistor, for example, set it to 2kΩ. If you do not know the resistor's value, start at the highest range (usually 2MΩ) and work downward.

The reason for starting high is that if you select a range too low for the resistor's actual value, the multimeter will display "1" or "OL" (overload) and you will not get a useful reading. Starting high and stepping down gives you a clearer picture of what the resistor actually is.

Remove the resistor from the circuit

Before you test, desolder or remove the resistor from the circuit board or breadboard. A resistor still connected to other components will give you a false reading because the multimeter will measure the combined resistance of multiple paths, not just the resistor itself. This is one of the most common mistakes when testing components.

If the resistor is soldered to a board and you do not want to desolder it, you can still get useful information by testing it in place — a very low reading (close to zero) usually means the resistor is shorted, and an infinite reading usually means it is open. But for a precise measurement, desoldering is the only reliable way.

Touch the probes to each end of the resistor

Hold the resistor steady or place it on a non-conductive surface. Touch one multimeter probe to each end of the resistor. It does not matter which probe touches which end — resistance is the same in both directions. Hold the probes firmly in place for a second or two while the multimeter takes the reading.

The display will show a number. If it shows a value close to the resistor's rated value (within about 5 to 10 percent), the resistor is good. For example, a 470Ω resistor might read 465Ω or 480Ω, and both are acceptable. If the reading is significantly different — say a 470Ω resistor reads 600Ω — the resistor has drifted out of tolerance and should be replaced.

Interpret the reading: good, bad, or uncertain

A good resistor reads a value close to its color-band rating. Most resistors have a tolerance of 5 or 10 percent, so a 1kΩ resistor rated at 5 percent tolerance should read between 950Ω and 1050Ω. Check the color bands or the component's datasheet to find the tolerance.

A shorted resistor reads zero or very close to it (usually under 1Ω). This means the resistor has failed and is no longer limiting current. A shorted resistor will cause excessive current to flow through that part of the circuit and can damage other components.

An open resistor reads infinity, displayed as "OL" or "∞" on the multimeter. This means the resistor has broken internally and no current can flow through it at all. An open resistor will stop current from reaching the rest of the circuit.

If your reading is wildly different from the rated value but not zero or infinity — for example, a 470Ω resistor reads 10kΩ — try a lower ohms range. You may have started on a range too high to give you a precise reading. Step down one range at a time until you get a stable, readable number.

Verify the resistor's color bands match the reading

If you are checking a resistor you pulled from a bin or a bag without a label, you can use the color bands to predict what the reading should be. Resistor color bands follow a standard code: each color represents a digit, and the bands are read left to right. A resistor with brown-black-red bands, for example, is 1000Ω (1kΩ).

Online resistor color code calculators are available if you do not remember the code. Photograph the resistor under good light, note the color order, and plug it into a calculator to get the expected value. Then compare that expected value to what your multimeter reads. If they match within tolerance, the resistor is good.

What to do if a resistor fails the test

If a resistor reads zero, infinity, or far outside its tolerance range, it has failed and should be replaced. Remove it from the circuit and install a new resistor with the same value and wattage rating. The wattage is usually printed on the resistor itself or listed in the circuit diagram — common values are 1/4 watt, 1/2 watt, and 1 watt.

If you replaced a resistor and the circuit still does not work, test the other components in that section of the circuit. A failed resistor sometimes indicates that another component failed first and caused the resistor to overheat and burn out.

Frequently Asked Questions

Do I need to power off the circuit before testing a resistor?

Yes. Always turn off power to the circuit and wait a few seconds before testing. Even if the resistor is still connected, testing a live circuit can damage your multimeter or give you a dangerous shock. If you are testing a resistor that is still soldered in place, power off first.

What if the multimeter shows a reading that keeps changing?

A changing reading usually means the probes are not making solid contact with the resistor ends. Press the probes more firmly and hold them steady for a full second. If the reading is still unstable, the resistor may be partially broken internally, which is a sign it should be replaced.

Can I test a resistor while it is still soldered to the board?

You can get a rough answer — zero or infinity tells you the resistor is definitely bad — but you will not get an accurate value because other components in the circuit affect the reading. For a true measurement, desolder the resistor first. If desoldering is not practical, note that an in-circuit reading that is much higher than expected often means the resistor is open.

Why does my resistor read higher than its color bands say?

Resistors drift over time, especially if they have been exposed to heat or high current. A resistor rated at 1kΩ might read 1.1kΩ or 1.2kΩ after years of use. If the reading is still within the tolerance printed on the resistor (usually 5 or 10 percent), it is still good. If it is far outside tolerance, replace it.

What does it mean if the multimeter shows "OL" on every range?

OL means overload or out of range. If you see it on the highest ohms range (usually 2MΩ), the resistor is open — it has failed and no current can flow through it. If you see OL on a lower range, switch to a higher range to get a readable number.