What a multimeter tells you about a resistor
A multimeter measures the resistance of a component by sending a small electrical signal through it and reading how much that signal gets blocked. A working resistor will show a reading close to its rated value — the number printed on it or listed in a schematic. A broken resistor typically reads either infinity (meaning the circuit is completely open) or zero (meaning it has no resistance at all), or sometimes a value wildly different from what it should be.
The test takes about 30 seconds per resistor and requires no power source. You can test a resistor while it is still soldered to a circuit board, though the reading may be affected by other components nearby. The most reliable results come from testing a resistor that has been removed from the circuit entirely.
Key Takeaways
- Set your multimeter to the resistance setting (marked with the omega symbol Ω) and choose a range that matches the resistor's rated value.
- Touch one probe to each end of the resistor and read the number on the display — it should be close to the printed value, within the resistor's tolerance rating.
- A reading of infinity means the resistor is open (broken), and a reading of zero means it is shorted (also broken).
- Remove the resistor from the circuit board before testing for the most accurate reading, since other components can interfere with the measurement.
Setting up your multimeter for resistance testing
Turn your multimeter on and locate the dial or menu. Look for the symbol that looks like the Greek letter omega (Ω), which stands for ohms — the unit of electrical resistance. On an analog multimeter, you will see a physical dial with multiple resistance ranges marked (usually ×1, ×10, ×100, ×1000, ×10000). On a digital multimeter, you will see a button or menu option labeled "Ohms" or "Ω".
Choose a range that matches the resistor's rated value. If the resistor is marked 470Ω, start with the ×100 range. If it is marked 10kΩ (10,000 ohms), use the ×1000 range. If you are unsure, start with a higher range and work downward — starting too low can damage some multimeters. Digital multimeters often have an "auto" setting that picks the range for you, which is the safest choice if your meter has one.
Testing the resistor step by step
Disconnect the resistor from the circuit board if possible. Desoldering it takes a few minutes with a soldering iron and solder wick or a desoldering pump, but gives you a clean reading. If you cannot remove it, you can still test it in place — just know that the reading may be lower than expected because other components in parallel with the resistor will affect the measurement.
Touch one probe (red or black, it does not matter for resistance) to one end of the resistor and the other probe to the other end. Hold them steady for a second or two. On an analog meter, watch the needle move across the scale. On a digital meter, read the number that appears on the display. The reading should be close to the resistor's rated value — usually within 5 to 20 percent, depending on the resistor's tolerance rating (often printed as ±5% or ±10%).
If the reading is within tolerance, the resistor is good. If it is way off, zero, or infinity, the resistor has failed and should be replaced.
Understanding what the reading means
A normal reading is one that matches the printed value within the resistor's tolerance. A 470Ω resistor with ±5% tolerance should read between 446 and 494 ohms. A 10kΩ resistor with ±10% tolerance should read between 9000 and 11000 ohms. If your reading falls in that range, the resistor is working.
A reading of infinity (shown as "OL" on digital meters, meaning "overload", or a needle that does not move on analog meters) means the resistor is open — the internal element has broken and no current can flow through it. This resistor must be replaced.
A reading of zero or near-zero means the resistor is shorted — its internal resistance has collapsed and it is acting like a wire. This also requires replacement. A resistor that reads zero will get very hot when power is applied and can damage the circuit around it.
A reading that is way too high or way too low — but not infinity or zero — usually means the resistor is degraded. Some resistors drift over time, especially if they have been exposed to heat or moisture. If the reading is more than 20 percent off from the rated value, it is safer to replace it.
Why testing in-circuit readings can be misleading
When a resistor is still soldered to a circuit board, other components connected in parallel with it will lower your reading. For example, if you are testing a 10kΩ resistor that has a capacitor soldered across it, the multimeter will read the combined effect of both components, not the resistor alone. This is why removing the resistor gives you the true picture.
If you cannot remove the resistor, test it twice: once with both probes touching the resistor leads, and once with one probe touching the resistor and the other probe touching the opposite side of the board (to measure the parallel path). If the second reading is much lower, something else is in parallel and affecting the result.
Common mistakes that give wrong readings
Touching the metal probes to your fingers while testing will add your body's resistance to the measurement. Always hold the probes by their plastic handles, not the metal tips. Your skin resistance is typically 1000 to 100000 ohms depending on moisture, so even a light touch can throw off the reading by a lot.
Using the wrong range on an analog meter will make the needle hard to read or cause it to peg at the end of the scale. If the needle does not move much, switch to a lower range. If it goes all the way to the right, switch to a higher range.
Testing a resistor while it is powered or while the circuit is powered will give false readings and can damage the multimeter. Always turn off power and wait a few seconds before testing. If the circuit has large capacitors, they may still hold a charge — discharge them by touching a screwdriver across their leads before you begin.
When to replace a resistor versus when to keep testing
If a resistor reads within tolerance, it is good — stop testing and move on. If it reads outside tolerance but the circuit is working fine, the resistor may be old but still functional. You can choose to replace it as preventive maintenance, or leave it alone.
If a resistor reads infinity or zero, replace it when ready. If a resistor reads way off and the circuit is misbehaving (no power, wrong voltage, component getting hot), replace it. If a resistor reads off but the circuit works, you have a choice: replace it to be safe, or monitor it and replace it only if the circuit fails later.
Frequently Asked Questions
Can I test a resistor while it is still soldered to the board?
Yes, but the reading may be inaccurate because other components in parallel will lower the measurement. For the most reliable result, desolder the resistor first. If you cannot, test it in place and note that the reading will be lower than the rated value.
What does OL mean on a digital multimeter?
OL stands for "overload" and means the resistance is too high to measure — usually infinity, which indicates the resistor is open and broken. This resistor needs to be replaced.
Do I need to turn off power before testing a resistor?
Yes, always. Testing a powered circuit can damage your multimeter and give false readings. Turn off the power supply, wait a few seconds, and discharge any large capacitors before you begin testing.
What if my reading is close but not exact?
Check the tolerance rating printed on the resistor (usually ±5% or ±10%). If your reading falls within that range, the resistor is good. If it is outside the range, the resistor has drifted and should be replaced.
Can a resistor fail partially?
Yes. A resistor can drift higher or lower over time due to heat, age, or moisture. It will still conduct electricity but at a different resistance than rated. If the reading is significantly off and the circuit is not working right, replace it.