How to Test a MOSFET With a Multimeter 2026: Find a Bad MOSFET Before It Burns Your Board

Last month, a technician brought me a dead inverter board. He had already fitted a new MOSFET. The board died again in ten minutes. He was angry. He said the new part was bad.

It was not the new part. The old fault was still on the board. One more MOSFET in the same group was shorted. He had tested only one part. He had missed the other one. That one mistake cost him a new part, a new board repair, and a lost customer.

This guide will stop that mistake. You will learn how to test a MOSFET with a simple digital multimeter. No costly tester. No lab. Just a meter, a few minutes, and the right steps in the right order.

This guide is for repair technicians and for home users who want to understand what their technician checks. You will learn the three pins of a MOSFET. You will learn three simple tests. You will learn what a good reading looks like, and what a bad reading looks like. You will also learn what a multimeter cannot prove. At the end, you will know when to replace a MOSFET, and when to look for a deeper fault first.

Trust nudge: SunTech Mall runs its own inverter and UPS repair service centre in Pakistan. Every day, we open inverters with MOSFET faults like these. We test them on the bench. We fix them. So what you read here comes from real bench work and from the guides linked at the end, not from guesses.

What You Will Learn in This Guide

• What a MOSFET is, in words a 10-year-old can follow
• The 3 pins: Gate, Drain, and Source, and how to find them
• The safety steps you must never skip
• Test 1: the body diode test, with good and bad readings
• Test 2: the gate isolation test, which catches a damaged gate
• Test 3: the gate switch test, which proves the MOSFET can turn on and off
• How to test a P-channel MOSFET (just swap the probes)
• How to read your results in one clear table
• Why testing on the board can fool you, and when to take the part off
• Why MOSFETs die in inverters, and what to check before you fit a new one
• What a multimeter cannot prove, said honestly

What Is a MOSFET, in Simple Words?

A MOSFET is a small electronic switch. Think of it like a tap for electricity. A small signal at one pin controls a large current through the other two pins. When the signal says "on", current flows. When the signal says "off", current stops.

Inverters use MOSFETs to turn battery DC power into AC power. They switch on and off thousands of times per second. That fast switching is hard work. Heat, overload, and wrong voltage can kill a MOSFET. When one dies, the inverter may stop working, blow a fuse, or burn a board track.

A MOSFET is not a fuse. A fuse is easy to see when it blows. A MOSFET can look perfect from outside and still be dead inside. That is why you must test it with a meter, not with your eyes.

Bonus Tip: Before you test anything, take a clear photo of the part number on the MOSFET and of the board around it. If you later need the same part, that photo saves you a second trip to the board.

The 3 Pins: Gate, Drain, and Source

Every MOSFET has three pins. Their names are:

• Gate (G): the control pin. A small voltage here turns the switch on or off.
• Drain (D): one side of the switch. In many power MOSFETs, the drain also connects to the metal tab on the back of the part.
• Source (S): the other side of the switch.

Most common power MOSFETs in inverters are N-channel types in TO-220 or TO-247 packages. In many of these, when you face the printed side of the part, the pins from left to right are Gate, Drain, Source. But this is not a rule for every part. Some parts use a different order.

Note: Never guess the pin order. Always check the datasheet for your exact part number before you test. A wrong pin guess gives wrong readings, and wrong readings lead to wrong repairs. If you cannot find the datasheet, compare with a known-good part of the same number.

Safety First: Read This Before You Touch the Meter

MOSFET testing is simple, but it is not risk-free. Follow these steps every time.

1. Switch the power off. Unplug the inverter. Remove the battery connections. Never test a MOSFET on a live board.
2. Discharge the big capacitors. Large capacitors on an inverter board can hold a painful charge after power off. Wait, then discharge them safely with a resistor, as your training teaches. If you do not know how to do this safely, stop and ask a trained technician.
3. Protect the gate. The gate of a MOSFET is very sensitive to static electricity. Touch a grounded metal surface first. Do not rub the part on cloth or plastic. Handle it by the body, not by the pins.
4. Use the right meter setting. This guide uses diode mode and resistance mode on a digital multimeter. Do not use a high-voltage insulation tester on a MOSFET. It can damage the gate.
5. Keep children away. A repair bench is not a play area. Hot parts, sharp pins, and charged capacitors are real hazards.

If any step feels unsafe, stop. A MOSFET costs little. Your safety costs everything.

What You Need

You need only three things:

• A digital multimeter with a diode test mode (the diode symbol on the dial) and a resistance mode.
• The MOSFET datasheet for your part number, for pin order and typical readings.
• A known-good MOSFET of the same type, if you have one, to compare readings. This is the fastest way to learn what "good" looks like on your own meter.

That is all. No special tester is needed for these three tests.

Test 1: The Body Diode Test

Inside most power MOSFETs there is a built-in diode between the Drain and the Source. It is called the body diode. It conducts in one direction only. This test checks that diode.

For an N-channel MOSFET, follow these steps:

1. Set the meter to diode mode.
2. Put the red probe on the Source and the black probe on the Drain.
3. Read the meter. A good MOSFET shows a forward voltage drop, typically about 0.4 to 0.7 volts. Some power MOSFETs read a little higher, near 1 volt. Your datasheet is the final word.
4. Now swap the probes: red on the Drain, black on the Source.
5. Read the meter again. A good MOSFET shows OL (open circuit, or overload). That means the diode blocks current in this direction, as it should.

How to read Test 1:

• One direction shows about 0.4 to 0.7 volts, the other shows OL: the body diode is good.
• Both directions show about 0.00 volts, or a very low reading: the Drain and Source are shorted. The MOSFET is bad.
• Both directions show OL: the body diode is open, or your probes are on the wrong pins. Check the pin order and test again. If the reading stays OL both ways, treat the part as bad.
• A reading that jumps around and never settles: clean the pins, press the probes firmly, and test again. A dirty pin gives a dirty reading.

Note: Some guides show the probes the other way round for the first reading. They are testing the same diode from the other side of the switch test, not a different diode. What matters is this: one direction must show the diode drop, and the other direction must show OL. If you get a drop in both directions, or OL in both directions, something is wrong.

Test 2: The Gate Isolation Test

The gate of a MOSFET is insulated. It must not pass DC current to the other pins. If it does, the gate is damaged, and the MOSFET cannot be trusted.

Follow these steps:

1. Set the meter to resistance mode (the ohm symbol).
2. Measure between Gate and Source. A good MOSFET shows a very high resistance, often OL or many megaohms.
3. Measure between Gate and Drain. Again, a good MOSFET shows a very high resistance, often OL.

How to read Test 2:

• Both readings very high or OL: the gate is isolated. Good.
• Either reading low, for example a few ohms or a few hundred ohms: the gate is damaged. The MOSFET is bad. Replace it.
• A reading that slowly climbs toward OL is normal on some meters. The meter is charging the tiny gate capacitance. Wait a few seconds and let it settle.

This test is quick, but it catches a common fault. A MOSFET with a damaged gate may still show a normal body diode in Test 1. If you skip Test 2, you can miss it.

Test 3: The Gate Switch Test (Can It Turn On and Off?)

Tests 1 and 2 check for damage. Test 3 checks the real job of a MOSFET: can it switch?

For an N-channel MOSFET, follow these steps:

1. Set the meter to diode mode.
2. Discharge the gate. Briefly short the Gate to the Source with a probe or a small wire. This turns the MOSFET fully off.
3. Put the black probe on the Source and the red probe on the Drain. The meter should show OL. The switch is off.
4. Now charge the gate. Keep the black probe on the Source. Lift the red probe from the Drain, touch it to the Gate for a second, then bring it back to the Drain.
5. Read the meter. A good MOSFET now shows a low reading, near zero, or continuity. The switch is on. The gate charge turned it on.
6. Discharge the gate again. Short the Gate to the Source once more.
7. Measure again with black on the Source and red on the Drain. The meter should return to OL. The switch is off again.

How to read Test 3:

• OL, then low, then OL again: the MOSFET switches on and off. Good.
• Always OL, never low: the gate may be dead, or your meter may not give enough voltage to turn this MOSFET on. Read the warning below before you condemn the part.
• Always low, even after you discharge the gate: the MOSFET is stuck on, or shorted. It is bad.
• Low in step 3 before you charge the gate: the MOSFET was already on, or it is shorted. Discharge the gate fully and repeat. If it stays low, it is bad.

Note: Some digital multimeters give only about 2 to 3 volts in diode mode. Some power MOSFETs need more gate voltage to turn fully on. If Test 3 shows no switching, but Tests 1 and 2 pass, do not condemn the part on Test 3 alone. Try resistance mode for the same steps, compare with a known-good part of the same number, and check the driver circuit on the board. A meter test finds clear faults. It does not prove every detail of a part.

P-Channel MOSFET: Swap Every Probe

A P-channel MOSFET works the same way, but with opposite polarity. The test steps are the same. You only swap the red and black probes at every step.

• Body diode test: red on the Drain, black on the Source gives the diode drop for a P-channel part. Swap the probes and you should see OL.
• Gate switch test: use the black probe to charge the gate, and keep the red probe on the Source side, mirroring the N-channel steps.
• Gate isolation test: the same rule applies. Very high resistance, or OL, is good. A low reading is bad.

If you are not sure whether your part is N-channel or P-channel, check the datasheet first. Testing with the wrong polarity gives confusing readings and wastes your time.

Bonus Tip: Write "N" or "P" on a small label when you remove a MOSFET from a board. When you have five parts on the bench, that one letter stops a costly mix-up.

How to Read Your Results: One Clear Table

If any row says "bad", the part is not safe to reuse. If all rows say "good", the part passes the meter tests. But remember the honest limit below: passing a meter test is not a full guarantee.

Testing on the Board vs Off the Board

This is where most beginners go wrong.

Inverters often use several MOSFETs in parallel. They share the same Drain and Source connections. If one MOSFET in the group is shorted, the whole group looks shorted on the meter. You cannot tell which one is bad while they are all soldered in place.

Other parts on the board can also fool your meter. A low-value resistor, a diode, or a transformer winding in parallel with the MOSFET changes your reading. A reading that looks bad may be the board, not the MOSFET.

The rule is simple:

• A clear short on the board tells you there is a fault in that group. It does not tell you which part is bad.
• For a sure result, remove the MOSFET from the board, or lift one leg where that is safe and practical, and test it alone.
• Test every MOSFET in a parallel group, one by one. The story at the start of this guide happened because only one part was tested.
• Compare with a known-good part of the same number. Same meter, same steps, same readings. A difference tells you the truth fast.

Note: Desoldering needs care and the right tools. If you have never desoldered a power part, do not learn on a customer's board. Heat damage from a bad desolder job can create a new fault.

Why MOSFETs Die in Inverters

A MOSFET rarely dies alone. Something usually kills it. If you fit a new part without fixing the cause, the new part dies too. Check these common causes:

• Heat. A loose heatsink, missing thermal paste, a blocked fan, or a dusty case makes the MOSFET run hot. Heat is the slowest killer, and the most common one.
• Overload. A load larger than the inverter can handle pushes too much current through the MOSFETs. The fuse may not save them in time.
• Gate drive fault. The small circuit that drives the gate must give the right voltage at the right speed. A weak driver, a burnt gate resistor, or a bad driver IC can leave the MOSFET half-on. A half-on MOSFET heats up fast and dies.
• Voltage spike. A bad battery connection, a loose wire, or a lightning surge can push voltage past the MOSFET rating.
• Wrong replacement part. A part with the same package but a lower voltage or current rating, or a different pin order, will fail early. Always match the full part number, not just the package.
• Shorted part nearby. A shorted diode, capacitor, or transformer winding on the same rail can kill a new MOSFET in seconds. Test the parts around the MOSFET before you power the board again.

Bonus Tip: After a MOSFET failure, always check the gate resistors and the driver section before you fit the new part. Five minutes of checking can save a second dead MOSFET.

What a Multimeter Cannot Prove

This part matters. A multimeter is a great first test. It is not a full test.

A multimeter cannot prove:

• The exact on-resistance of the MOSFET at full gate voltage.
• How fast the MOSFET switches under real load.
• Whether it can handle full voltage and full current without breaking down.
• Whether it fails only when hot, or only after running for an hour.

A MOSFET can pass all three tests in this guide and still fail under load. That happens when the damage is small, like higher leakage or slower switching. If your meter tests pass but the board still fails, the next step is a live voltage test or a bench test under load, done by a trained technician with current limiting.

Do not sell a meter test as a full guarantee. Say what it is: a fast, honest check that finds clear faults.

Before You Fit a New MOSFET: 6 Checks

1. Match the part number. Same number, same package, same or better voltage and current rating. Do not guess a "close" part.
2. Check the pin order. Compare the new part's datasheet with the old part. Same package does not always mean same pin order.
3. Check the heatsink and thermal paste. Clean the old paste. Use fresh paste. Tighten the part firmly, but do not crack the package.
4. Check the gate resistor and driver. If the gate resistor is burnt or open, replace it. If the driver is suspect, test it before power-up.
5. Check the parts around it. Test the diodes, capacitors, and other MOSFETs in the same group. One hidden short kills a new part fast.
6. Power up with care. After fitting, power the board with no heavy load first, and watch for heat, smell, or a blown fuse. If anything feels wrong, switch off at once.

If any check fails, stop and fix it first. A new MOSFET is not a repair. A repaired cause is a repair.

Where SunTech Mall Fits In

SunTech Mall is Pakistan's online store for original solar inverter and UPS spare parts. We also run our own repair service centre. That means we see both sides: what buyers choose, and what fails on the bench.

If your test shows a dead MOSFET, start with the exact part number. Do not buy by package alone. Check the live price on the website for the model you want, because prices change and this article does not fix a price.

MOSFETs to check (live, in stock at check time): For a high-power TO-247 part, see the HY3215 N Channel Mosfet To-247. For a high-current TO-220 part, see the NCEP 85T16 85V 160A N-Channel MOSFET To-220. To browse the full range, see the MOSFET category. Check the live price on the website for each model, because prices change.

Inverters that use these parts: MOSFET faults often show up as fault codes or as a dead inverter. For Inverex codes, read Inverex Nitrox Fault Codes 2026. For Fronus codes, read Fronus PV 7200 Error Codes 2026. If your inverter will not turn on at all, read Solar Inverter Not Turning On 2026. For a related switching-part fault, read Genuine Chopper Transformer for Hybrid Solar Inverter. For battery-side context, read Lithium Battery vs Tubular Battery 2026.

Related inverters: See the Inverex Nitrox 6KW-3Ph On-Grid Solar Inverter and the Inverex Yukon II 3500W – 24V Solar Inverter. Check the live price on the website for each model, because prices change.

Need help matching a part? Contact SunTech Mall through the website, and share your MOSFET part number and your inverter model. Our team can help you match the part before you buy. You can also reach us on WhatsApp at 0306-2244820 for help with your inverter model and part number.

Frequently Asked Questions

### Can I test a MOSFET without removing it from the board?

You can test it, but you cannot always trust the result. Other parts in parallel change the reading. A short on the board may come from a different part in the same group. For a sure result, remove the MOSFET and test it alone.

### What does OL mean on my multimeter?

OL means overload, or open circuit. It means the meter sees no path for current in that direction. In the body diode test, OL in one direction is good. OL in both directions is bad, unless your probes are on the wrong pins.

### My MOSFET shows 0.00 volts in both directions. Is it dead?

Yes, in most cases. A reading near zero in both directions means the Drain and Source are shorted. But first check that your probes are on the right pins, and that the part is off the board. A short on the board can fake this reading.

### My MOSFET passes the diode test but fails the switch test. Is it bad?

Not always. Some meters give too little voltage in diode mode to turn the MOSFET on. If the diode test and the gate test pass, compare with a known-good part of the same number on the same meter. If the known-good part also fails the switch test, your meter is the limit, not the part.

### Can a MOSFET look good but still be bad?

Yes. A meter test finds clear faults: shorts, open diodes, and damaged gates. It cannot prove full switching speed, full current handling, or failure when hot. If the board still fails after a passing meter test, check the driver circuit and test under load with care.

### Do N-channel and P-channel MOSFETs test the same way?

The steps are the same, but the probe colours swap. For a P-channel part, swap red and black at every step. If you are not sure which type you have, check the datasheet before you test.

### Should I replace all MOSFETs in a parallel group when one is bad?

Test each one alone first. Replace only the bad ones, but check the cause for the whole group. If one part died from heat or a driver fault, the other parts in the group may be weak too. When in doubt, compare all of them with a known-good part.

Final Thoughts

Do not replace a MOSFET because it looks burnt. Do not keep a MOSFET because it looks fine. Test it. Three tests, in order, take five minutes. They save parts, boards, and customers.

Test the body diode. Test the gate. Test the switch. Then test the parts around it. Then fix the cause, not just the part.

If this guide helped you, share it with a technician who still replaces parts by guesswork. One good test can save a whole board.

Published: 27 October 2026 | Updated: 27 October 2026 | SunTech Mall — Charsadda/Tangi Office, Tangi, Charsadda, Khyber Pakhtunkhwa

Sources: MOSFET multimeter testing guides — https://www.engineersgarage.com/testing-mosfet-part-16-17/ ; https://www.wellpcb.com/blog/pcb-test/testing-a-mosfet/ ; https://www.homemade-circuits.com/how-to-check-mosfet-using-digital/ ; https://www.xmagelec.com/blog/how-to-test-mosfet-optocoupler-ntc-thermistor-multimeter ; NCEP85T16 datasheet (85V, 160A, RDS(on) under 3.8 mΩ at 10V) — https://hifau.com/uploads/file/pdffiles/NCEP85T16.pdf ; DMM voltage limit discussion — https://www.eevblog.com/forum/beginners/how-to-test-mosfet-with-a-multimeter/