REVIEW · MULTIMETERS
Klein Tools MM400 Review
A solid auto-ranging CAT III meter with temperature and capacitance, but not True RMS, and Klein has now discontinued it.
By the OhmSavvy Research Team ·
| Model | Klein Tools MM400 |
|---|---|
| Accuracy | Auto-ranging (not True RMS) |
| Safety rating | CAT III 600V |
| Auto-ranging | Yes |
| Best for | Tradespeople and homeowners doing their own electrical work |
| Review status | Desk research, not bench-tested |
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The verdict
Klein's MM400 is an auto-ranging, CAT III 600V meter with a 4,000-count backlit display that also reads temperature, capacitance, frequency and duty cycle. It is not True RMS, and Klein lists it as no longer in production (the MM420 replaces it), so buy it only at a clear discount or look at a True RMS meter for HVAC and variable-speed drive work.
Consider it if
- ✓You need CAT III 600V for panel and building-wiring work but don't want to pay Fluke prices
- ✓You want temperature, capacitance and frequency in one auto-ranging meter
- ✓You want a brand with an established reputation among tradespeople
- ✓You find one at a clear discount from a seller you trust, knowing it is discontinued
Skip it if
- ✕You want a meter that fits in a pocket — the MM400 is bulkier
- ✕You work on variable-speed drives or electronic loads, where True RMS matters
- ✕You want a model that is still in production: Klein has replaced it with the MM420
- ✕You want a non-contact voltage detector or low-impedance mode built in
The trade-off: The MM400 shares the Fluke 117's CAT III 600V rating but not its True RMS measurement, low-impedance mode or NCV. Klein's own feature chart reserves True RMS and low impedance for the MM700. It is a capable general-purpose meter for sinusoidal mains work, but it is discontinued, so it only makes sense at a clear discount.
Key specifications
- Accuracy
- Auto-ranging (not True RMS)
- Safety rating
- CAT III 600V
- Auto-ranging
- Yes
- Best for
- Tradespeople, home electrical work
Checked against the manufacturer's official product page or datasheet and the official Amazon listing when this model was added to our guide.
Strengths and limitations
Reasons to buy
- +Drop-rated to 1 m (3.3 ft) for job-site use
- +Measures temperature, capacitance, frequency and duty cycle
- +Backlight, min/max and range hold
Reasons to avoid
- −Not True RMS, so readings on non-sinusoidal signals are less accurate
- −Discontinued by Klein (replaced by the MM420)
- −No low-impedance mode or NCV
Performance assessment
We haven't bench-tested this unit. This assessment is based on its published specifications and on what those specifications mean in practice. How we research.
Measurement range
Klein rates it to 600V AC/DC, 10A AC/DC and 40 MΩ, with temperature from 0°F to 1000°F, capacitance to 200 µF and frequency/duty cycle from 1 Hz to 50 kHz on a 4,000-count backlit display. That covers most household electrical and appliance checks.
Safety and accuracy
CAT III 600V suits household panels and building wiring. It is not True RMS, so on distorted waveforms from variable-speed drives, LED drivers and other electronic loads its AC readings can be off; on ordinary sinusoidal mains it reads correctly.
Why True RMS matters, with numbers
An average-responding meter like the MM400 measures the average of the rectified waveform and scales it by 1.11, the ratio that holds for a pure sine. Feed it a square wave and that ratio is wrong: the reading comes out about 11% high. The current drawn by a rectifier-input power supply, a peaky waveform, typically reads low instead, sometimes by a third or more depending on how peaky it is. On a 120 V outlet feeding a resistive heater the MM400 is fine; on the output of a variable-speed drive, treat its AC reading as approximate.
What 4,000 counts means in practice
A 4,000-count display shows up to 3,999 before it changes range. With the usual 4,000-count range steps (4 V, 40 V, 400 V), 120 V mains reads to 0.1 V, which is all a household check needs. It also means a 4.2 V reading jumps to the next range and loses a digit, so the 6,000-count Fluke 117 holds one more digit of resolution across a band of everyday voltages.
Temperature and capacitance as practical extras
The thermocouple temperature function covering 0°F to 1000°F lets you check a supply-air register, a pipe or an appliance element without a separate thermometer. The 200 µF capacitance ceiling covers most motor-run capacitors, which are usually in the single to tens of microfarads, but not larger motor-start capacitors that can run into the hundreds. Always discharge a capacitor before measuring it.
Build, power and current fusing
Klein lists a 1 m (3.3 ft) drop rating, a weight of 232 g and two AAA batteries. Its replacement-parts list includes a 10A fuse and a 500mA fuse, so the current inputs are protected, and a blown fuse is a parts job rather than a dead meter. Use fuses of the exact rating Klein specifies; a lower-rated glass fuse defeats the CAT III rating.
Buying a discontinued meter
Klein says the MM400 is no longer in production and points buyers to the MM420. Amazon still lists it, but through third-party offers rather than a standard buy box, so check the seller, the condition and the return terms. If the price is close to a current True RMS meter, the newer meter is the better buy.
Alternatives to consider
Fluke 117
The same CAT III 600V rating plus True RMS, a low-impedance mode and built-in non-contact voltage detection, at a premium.
Fluke 115
True RMS and CAT III 600V without the 117's electrician extras, for general field work.
Milwaukee 2235-20
If most of your checks are current draw, a clamp meter avoids opening the circuit.
KAIWEETS KM200s
Cheaper, but rated CAT II only: fine for outlets, not panels.
Where to buy the Klein Tools MM400
We link to the official Amazon listing. Prices change often, so check the current price at the retailer before you buy.
Frequently asked questions
Is the Klein MM400 True RMS?+
No. Klein's multimeter feature chart lists True RMS only for the MM700 in that range, not the MM300, MM400 or MM600. The MM400 reads ordinary sine-wave mains correctly but can misread distorted waveforms from drives, dimmers, LED drivers and other electronic loads, so it is a poor choice for HVAC inverter work.
Klein Tools MM400 or Fluke 117?+
Both are CAT III 600V and auto-ranging, but only the Fluke 117 is True RMS, and it adds a low-impedance mode and built-in non-contact voltage detection. The MM400 adds temperature readings with its included thermocouple. For electrical work the 117 is the better meter; the MM400 only makes sense at a clear discount, especially now it is discontinued.
Why does Klein's reputation matter if it isn't a dedicated test-equipment brand?+
Klein Tools built its name on hand tools that tradespeople carry and depend on daily, such as pliers, strippers and fish tapes. That is a different track record from a meter-specific brand like Fluke, but it shows the company knows what durability means in a tool bag. The MM400's 1 m drop rating reflects that thinking.
Has the Klein MM400 been discontinued?+
Yes. Klein's product page says the MM400 is no longer in production and names the MM420 as its replacement. Amazon still offers it through third-party sellers rather than a standard buy box, so check condition, seller and return terms. A discontinued meter remains safe to use if it is genuine and its fuses and leads are in good order.
Can the Klein MM400 test HVAC capacitors?+
Often, yes. Klein rates its capacitance function to 200 µF, which covers most run capacitors on air-conditioner condensers and furnace blowers. Larger start capacitors can exceed that range. Switch off power, discharge the capacitor safely, and disconnect at least one lead before measuring, or the reading and your safety both suffer.
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