A resistor that looks fine might not be working

A resistor is a component that slows down electrical current. Over time, resistors can fail — they burn out, crack, or their internal material degrades. You cannot tell by looking whether a resistor still works. You need a tool called a multimeter to measure its resistance and find out.

The reading you get tells you whether the resistor matches its rated value. If it reads far too high, too low, or shows no reading at all, the resistor has failed and needs to be replaced. This matters because a bad resistor can cause a circuit to malfunction, produce heat, or stop working entirely.

Key Takeaways

  • A multimeter is the only practical way to check whether a resistor works; visual inspection cannot tell you if it has failed internally.
  • Set your multimeter to the ohms (Ω) setting that matches or exceeds the resistor's rated value before you measure.
  • Touch one probe to each end of the resistor and read the number on the display; it should be close to the rated value printed on the resistor.
  • A reading of zero, infinity, or a value far outside the normal range means the resistor has failed and should be replaced.

What you need to check a resistor

You need a digital multimeter (also called a DMM or ohmmeter). These cost between $10 and $30 for a basic model at any hardware store or online retailer. Look for one with a dial or digital display and a setting marked with the Greek letter omega (Ω), which represents ohms — the unit of electrical resistance.

You also need to know the resistor's rated value. This is printed on the resistor itself using color bands (colored stripes) or, on larger resistors, printed numbers. If you do not know the value, you can still test the resistor — a working resistor will show some resistance reading, while a failed one will show zero or infinity.

Remove the resistor from the circuit before testing it. A resistor still connected to other components can give you a false reading because the multimeter will measure the combined resistance of multiple paths.

How to set up your multimeter

Turn on the multimeter and look at the dial or menu. Find the ohms (Ω) setting. If your multimeter has multiple ohms settings — usually labeled ×1, ×10, ×100, ×1k, or ×10k — choose the one that matches or is just above the resistor's rated value. For example, if the resistor is rated 470 ohms, use the ×100 setting. If it is rated 10,000 ohms (10k), use the ×1k or ×10k setting.

If you do not know the rated value, start with the highest ohms setting and work your way down until you get a readable number on the display. This prevents you from overloading the meter on a low setting.

Some modern multimeters have an auto-range feature that picks the right setting for you. If yours has this, select it and skip the manual setting step.

Testing the resistor step by step

Hold the resistor in one hand or place it on a non-conductive surface like paper or plastic. Do not hold both ends of the resistor with your bare hands — your skin has resistance, and it will interfere with the reading.

Take the two probes from the multimeter (one red, one black) and touch one probe to each end of the resistor. It does not matter which probe touches which end. Wait one or two seconds for the display to stabilize and show a steady number.

Read the number on the display. If your multimeter has a dial, the needle will point to a number on the scale. If it is digital, the number appears on the screen. Write down what you see.

What the reading means

A working resistor will show a reading close to its rated value. "Close" usually means within 5 to 10 percent. For example, a resistor rated 1000 ohms might read anywhere from 950 to 1050 ohms and still be good. Resistors have a tolerance rating (often printed as ±5% or ±10%) that tells you how far off the reading can be and still be acceptable.

If the reading is zero or very close to zero, the resistor has failed — its internal material has burned out or shorted. If the reading is infinity (shown as "OL" or "∞" on the display), the resistor is open — broken inside or disconnected. Either way, the resistor needs to be replaced.

If the reading is far outside the tolerance range — for example, a 1000-ohm resistor reading 500 ohms or 2000 ohms — the resistor has degraded and should be replaced. Resistors that drift out of spec can cause circuits to behave unpredictably.

Why resistors fail

Resistors fail when they are exposed to heat, moisture, or electrical stress. A resistor rated for 1/4 watt that carries too much current will overheat and burn out. Moisture can seep into the component and cause corrosion. Age alone can cause the internal material to degrade, especially in older electronics.

If a resistor fails, the circuit it was protecting or controlling will not work correctly. A failed resistor in a power supply might allow too much current through, damaging other parts. A failed resistor in a sensor circuit might cause false readings. Testing resistors is a quick way to rule them out as the cause of a circuit problem.

When to test resistors in a circuit

If you are troubleshooting a device that stopped working, test resistors if the device has visible burn marks, a burnt smell, or if you suspect a component has failed. Start with resistors near the problem area — if a section of a circuit is not working, test the resistors in that section first.

You can also test resistors as a preventive step. If you are repairing an old device, testing resistors before you reassemble it can save you from putting a failed component back in and having the device fail again shortly after.

Frequently Asked Questions

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

Not reliably. Other components in the circuit will affect the reading. Desolder the resistor first or at least lift one end of it off the board so the multimeter only measures that one resistor, not the combined resistance of multiple paths.

What if my multimeter shows a reading that is slightly off from the rated value?

Check the resistor's tolerance rating, printed on the component or in the datasheet. A 1000-ohm resistor with ±10% tolerance can read anywhere from 900 to 1100 ohms and still be good. If the reading is within tolerance, the resistor is working.

Does the order of the probes matter?

No. Resistance is the same in both directions. You can touch the red probe to either end and the black probe to the other end — the reading will be the same.

What does "OL" mean on the display?

"OL" stands for overload or open circuit. It means the resistor is broken inside or the connection is severed. The resistor has failed and needs to be replaced.

Can a resistor fail partially and still work?

Yes. A resistor can drift out of its rated value and still conduct current. This is why checking the actual reading matters — a resistor that reads 2000 ohms when it should read 1000 ohms is degraded and should be replaced, even though it is not completely open.